You've posted on the Yamaha Synth website, in the MX49/MX61 Forum... We are going to assume you own a Yamaha MX that you are using with Cubase Pro 8... connected via USB.

You will need to setup your MX as an EXTERNAL INSTRUMENT so that it can "Return" Audio to Cubase Pro 8 in a fashion that allows you to use Steinberg's Virtual Studio Technology signal routing. This will allow you to route your MIDI Tracks to the MX and then have the audio return through an VST AUDIO LANE - you can set your Left-Right Markers, Mute Tracks you don't want to render as audio, and print Audio using the EXPORT > Audio Mixdown routine

What you will need:
MX49/MX61 connected to your computer via USB
Yamaha Steinberg USB Driver appropriate for your computer and operating system
MIDI DEVICE MANAGER (XML) voice list for the Yamaha MX

Setup MIDI Device Manager
Go to DEVICES > MIDI DEVICE MANAGER > Install Device > select the "Yamaha MX (YAMAHA)" script
Set the OUTPUT = Yamaha MX49/MX61 (Port1)
Click OK

Setup the MX as an External Instrument
Go to DEVICES > VST CONNECTIONS > EXTERNAL INSTRUMENT
Add External Instrument
Set it for 1 Stereo Return
Click on "Associate MIDI Device"
Select the "YAMAHA MX"
Click OK

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Your External Instrument tab will look like this once you select the Audio Device and Device Ports:

On the main Cubase Pro 8 window click on RACK INSTRUMENTS
The Yamaha MX will appear under EXTERAL PLUG-INS, select it.

"Do you want to create a MIDI track assigned to plugin "Yamaha MX"? Will appear... CREATE
Each new MIDI Track you create (using the ADD TRACK > MIDI) will need to be assigned to the same "Yamaha MX" plugin.

Notice a VST INSTRUMENTS FOLDER and a SubFolder for the MX VST was created, and a VST AUDIO LANE appears, then the MIDI track.
You may want to MUTE (M) the Audio Lane to prevent listening to both the direct and the return... This is your perogative... The Audio Lane's meter is stereo (thus it will have a left and a right meter)... the MIDI Track which is not audio, is neither Mono nor Stereo... only Audio can be mono or stereo!
The MIDI data is sent to the MX - the Audio Lane is the stereo audio return from the MX hardware.

When you want to render any of your MIDI tracks as audio go to FILE > Export > Audio Mixdown
Set the dialog box that appears carefully (please see your Steinberg documentation for the best way to set this up for your needs).
It is very important to set the LEFT-RIGHT Markers to define the region that will be rendered.
You can Mute Midi tracks you don't want in your audio rendering or you can render them all as a stereo mix.

Because the MX is a real synth (not virtual), it will render the wave in real time. If you are using effects, set the "tail" parameter appropriately so that effects that hang over (like reverb, or echoes) don't get chopped off.

Hope that helps... If you get stuck, post back here.