iRig Pads should work fine with Melodics in it's default setting – however it's easy to customise MIDI messages or switch to a new scene, which could cause issues.
If the pads aren't triggering samples or lighting up, try this:
Then press the bottom left pad
Boom, should be going now.
The next thing to try is a factory reset:
Make sure the iRig Pads is disconnected
While holding SCENE and FIX VEL, connect to your computer
Release SCENE and FIX VEL
Press SCENE or FIX VEL
And you should be ready to go!