On the Q9h you have to hit FN and then the spacebar to bring up a secondary menu. Give that a shot.
Nope.. that doesn't work.
What's frustrating is that I've previously somehow hit a right key combination as I've been typing something else and not trying to get the underscore, but suddenly see optional characters pop up.
The Motorola User Manual is rather useless... does not address the keyboard at all. They don't even have it in the index. :(
Fn > Shift to see the symbol menus. The underscore is the first symbol.
I know people love the Touch Pro 2's keyboard, even me, but I still say the Q9c/h/m/n has the best keyboard ever. I prefer it to a SideKick's (II is the only version I used) keyboard because my thumbs never hurt after a long day of e-mail and texting.