Frankly, I've no idea what's wrong.
Here's what I get in the Decal portion of the 'Mechlab:
I would show you what happens when I try to move the decal around, adjust its size, or select another decal/slot, but the problem is what happens: nothing at all. I can do whatever I want, but entering the decal placement screen doesn't let me place a decal. I can't even double-click to place an imaginary decal anywhere- the only things I can do are rotate the 'mech and hit Escape to exit.
You can clearly see that I have the MC necessary for a decal, and I know of no other reason why I might be prevented within the structure set out by the decal system, so..... help? Some kind of fix or even just a suggestion what's wrong?