Howdy - I have the full X2 and while the instrument define button is also grayed out, this is by design.
What you have to FIRST do is click on Edit>Preferences>MIDI>Devices and place check boxes on at least one OUTPUT device, then click Apply.
After that, go back to Instruments and you will see whatever comes up for what you checked for output showing up in the box on the left, and this also opens up the Define button for you to use.
While I do not know if this helps to actually get your Pro Audio INS files loaded/defined/whatever, at least you should be able to click the Define button following the above steps.
Hope that helps,
Bob Bone