Certainly on the Motif ES6 side of things, you can connect the Motif ES to your computer via a USB cable for MIDI communication.
You will need the Yamaha USB-MIDI driver for your computer type. This can be downloaded from the Official Yamaha Download site: http://download.yamaha.com

Select your country
Type in your MODEL NAME: Motif ES
When the SEARCH RESULTS appear select your model: Motif ES6
Find the USB-MIDI driver for your Mac Operating System. This must be installed on your computer. The download includes the instructions!

Setting up MainStage 3 - please consult their manual.

As to terms like LOCAL CONTROL = OFF or ON, this refers to the keyboard (Motif ES) communicating with the tone generator (Motif ES). When Local Control is ON, the keys trigger the tone generator (normal operation). When Local Control is OFF, pressing the keys on the ES will only generate MIDI data that travels OUT via the USB Port (assuming you are connecting to a computer and MIDI In/Out is set to USB). This allows you to decide in your computer software whether the key press will trigger the Motif ES, or some other tone generator.

Say you want the key presses not to trigger the Motif ES but instead, to trigger one of the soft-synths you may have on your computer... you would set the Motif ES to Local Control = OFF

If you want the key presses to trigger the Motif ES itself, you would leave LOCAL CONTROL OFF but use your computer software to route the MIDI signal back to the Motif ES on USB Port 1.

A PORT in MIDI is the equivalent of a MIDI IN plus a MIDI OUT. So in a USB cable multiple IN/OUT sets can be running simultaneously between devices. In your Motif ES for example:
PORT 1 is for IN and OUT to the Motif ES tone generator itself
PORT 2 and higher can be used for PLG150-series plugin boards, or for Remote Controlling your software, or you can connect one external device to the 5-pin MIDI jacks IN/OUT and address it discreetly from the computer!

Each PORT contains 16 MIDI channels. MIDI Channels are explained in your Owner's Manual. The word channel is used to simplify understanding. Like TV channels... you must tune the receiving device to a specific channel to get the NEWS broadcast by that station. Multiple stations can be broadcasting simultaneously but the local tv must be set to a specific channel to receive the specific broadcast.

In a MIDI system your Piano might be on channel 1, your Bass on Channel 2, your Guitar on Channel 3 and so on. The data will remain discreet on its own channel. Messages on a specific MIDI channel on address receiving devices TUNED into that channel - make sense.

So the Motif ES is 16 MIDI channels. in SONG or PATTERN mode you can place a different instrument on each of the 16 Channels and address each one individually.