This assumes that you are networked correctly and EasyRemote is connected to the usb DMX device:
1) Create the button in Easy Remote
2) Click the lock icon to make sure you are in "live" mode (lock should be closed).
3) Go to the User tab in MyDMX 2.0
4) Right-Click on a scene and select "Learn Easy Remote Command...". A "Waiting Command..." prompt will appear. If you aren't able to select "Learn Easy Remote Command", then MyDMX doesn't see that your iPad is synced.
5) Press the button on the iPad
6) The "Waiting Command..." window should go away and the button will be assigned.
This is pretty much the same procedure for hooking anything up.