As soon as you enter the Crawler you'll see they've setup Repulsor Fields. Luckily, the switches are right next to them. Just switch them off and in you go. There are no hostiles on this level.

You will, however, spot one odd person hanging out with a few robots. He'll come over and talk to you almost as soon as you see him.

If you talk nicely with Stiggs he can give you a lot of information and some good tips. After your conversation with Stiggs, search the area. This level is filled with supplies.

When you're ready to continue, just head on over to the door marked "Launch Pad" and climb up the ladder.

To your left you'll see a door marked Storage. The other door, with the warning above it, is the one we want. Head on through using sneak.

Up top in the glowing Repulsor Fields is the Mainframe. You can reach it by heading across the room and up through a set of stairs. Hopefully you have halfway decent hacking skills by this point, because the Mainframe is key to making this part of the mission much easier. Note: there are tons of enemies from this point on, including Soldiers, turrets, and robots galore.