Sorry we don’t know the item you are using... could you please describe what you’ve done so far?
Or at least how it is supposed to connect to the synthesizer?

You need to program the MODX Performance Part, to respond to BC.
Breath Control can be assigned to any number of parameters — what are you attempting to control?

The MRT-BC V2 needs to be plugged in correctly. I have seen the reviews of this product complain about the documentation, so I have to assume it is not sufficient to clearly define how to use the device. I hope you don't mind a few suggestions that may be below your knowledge level.

I also have to disclaim that I don't have one of these - so I am working on inference here.

Note that the most major part of the MRT-BC V2 that makes this a "Yamaha" compatible product is the inclusion of the breath controller interface that can plug into keyboards like the DX with a dedicated BC port. The port looks like a standard 3.5mm jack that is used for the smaller earphone jacks used on (some) cell phones and other consumer equipment.

The MRT-BC has a means of plugging its output into the DX7 to emulate the breath controller hardware originally used in the DX7 (and others).

For the MODX, you don't have to worry about this feature since MODX doesn't have a dedicated breath controller input. Instead MODX relies on a device to translate breath into MIDI for its breath controller functionality. The default MIDI CC# for MODX's BC is CC #2. The MRT-BC has a dial to set this configuration so the MRT-BC outputs data on CC#2.

I do have to step back and talk about the physical connection between the MRT-BC and your MODX. I've covered what not to use. Don't use the 3.5mm connection. As far as what you should use - the only connectivity the MRT-BC provides that is compatible with MODX is a 5-pin DIN MIDI connector. This is the connector you plug the MRT-BC into:

The only connection I would make between the MODX and the MRT-BC is a single 5-pin DIN MIDI cable and no other connections.

The MRT-BC has a USB connector. This is not for MIDI, the computer, or anything other than power. I would not plug this into MODX because it has no value in the setup to plug into MODX and also I do not know how much power the MRT-BC needs vs. how much the MODX's USB port would supply. It's not worth plugging up a MODX connector with an unapproved device just to supply power. What should be done is plugging in the USB cable to a power supply that has a "type A" connector to supply 5V to the MRT-BC device through a USB cable. The MRT-BC comes with a "USB Adapter" which is a power adapter and this should be used for supplying power instead of any device.

That covers the basic "plugging it in" setup.

Now to your MODX configuration.

It would be helpful to know if you have MODX connected to a computer or iPad using the "USB TO HOST" connector and if you use this USB+MIDI connection or not. I am going to assume you do not. If you do, that's fine - but there are some extra software setup steps to do which I will not cover yet.

Assuming no USB-MIDI connection, you need to make sure that your MIDI-IO Setting is set to "MIDI" and not "USB". This allows for MODX to directly "talk" to the MRT-BC connected to the external MIDI port on the back of MODX.

Back to the MRT-BC

You have choices what to set the CC# dial to on the MRT-BC. Your choices are 1, 2, 7, or 11. FYI - CC#2 is the breath controller. However, you may have trouble finding a Performance that is already programmed to use the breath controller. Therefore, I would set the CC# to something that will have more impact. Set it to CC#1 for modulation. This will make your breath controller "virtually" move the modulation wheel. Almost every Performance does something audible with the mod wheel.

The MRT-BC has a second knob for MIDI Channel(s). Set it to CH1. The other 2 settings are CH1+CH2 and CH1+CH2+CH3. Something like the mod wheel (CC#1) or breath controller (CC#2) or even expression (CC#11) would work well with the CH1 setting. If you set the CC to CC#7, then this will "act" differently than the others. It will adjust the volume of the channel(s) set by the second knob. Assuming your keyboard is in Multi-Channel MIDI mode (and not Single or Hybrid) - then CC#7 will change the volume of the Parts matching the channels. One setting, only Part 1 (CH1) - one setting Parts 1 and Part 2 (CH1+CH2), the last setting Parts 1, 2, and 3 (CH1+CH2+CH3). If you really wanted to use this breath controller to change volumes of all part volumes together - and didn't want to use expression (CC#11) - and didn't want to use the modulation matrix (programming inside MODX) - then you would set your MODX's MIDI mode to "Single" or "Hybrid", set the Transmit/Receive channel to channel 1, then set the MRT-BC to "CH1".

Generally, you should just set the MRT-BC to CH1 when using this with MODX. Although there are some novel uses where you would change this.

After you get the basics working - you can change the CC# to CC#2. MIDI Channel knob to CH1. Then use the "Mod/Control" -> "Control Assign" source/destination matrix where the source is "BC" (Breath controller) and the destination is the parameter you want to offset by using the breath controller.