I finally got Duke Nukem Forever installed on my new computer and started playing it last night. Right away, I got stuck in the vents off of Duke's training room. I can see a ladder going down from a grate in the floor. Presumably, this is where I need to go next. How do I open the grate to descend the ladder? Or, is there something else I should be seeking? Thanks.
When you enter the vents, you'll see an alien scurrying across the T-junction in front of you, running of to your right; follow it to the right. You'll see it again at the end, darting around the corner to the left. Follow it, and then a pipe will explode and start spouting flames; you'll see the alien dart out of sight on the other side of the flames.
Backtrack from there until you find an intersection that you'd passed, leading off to your right (was your left when you first passed it); take that, and it will lead you quite conveniently past the fire, and then you'll head up a ramp.
From there, I don't remember if there's a grate or not, but I'm pretty sure there's no ladder around. If there is a grate, either mouse click to punch it into oblivion, or space to sort of cinematically tear it asunder. That should get you on track again. If you're still having trouble with this, I'll load up the game when I get home and run through that part again, then update my answer with a clearer walkthrough of this portion.
Duke Nukem is definitely what I call a "railroad" game -- there is precisely one path to get you from A to B, and there's no room to deviate from that path in the slightest. This gives you an invaluable rule of thumb: If you get stuck at a dead end, backtrack until you find the route you were supposed to take initially; in other words, even if there's an enticing ladder on the other side of the grate, if you can't easily figure out how to get past the grate, you're probably supposed to go a different route anyway.