Wow, it's been a while since someone's asked a real singleplayer question in here.
Welcome to the DBB!
Now to your question: do you remember how the shuttle pilot messaged that you needed some sort of "diversion" to get Sweitzer transferred to minimum security? Well, you need to find a way to create that diversion. When you're in the maximum security area, look around; you'll eventually see a destroyable grate with a sign above it saying "Service Tunnel." Go through there and head down. You'll eventually come to a room with two levers and a window looking into another room with a vat of acid in it. The lever on the left creates a chemical capsule and summons a maintenance bot to pick it up; the right lever destroys the maintenance bot, dropping the chemical capsule if the bot has already picked it up. Now, notice the signs in front of you? "Warning: Keep chemicals away from acid!" Hmmm...you have a chemical capsule, you have a vat of acid...maybe you should try pulling some Mad Science?
Anyway, after you've created the distraction, you'll need to head back up to minimum security to find Sweitzer's new cell, and then find a way to disable the forcefield so that the rescue shuttle can get in to pick him up. Note: if you're ever having problems finding something, just hit F4 and release your Guide-bot; most of the time, he'll be able to take you where you need to go. If you're still having problems with this or any other level, check out
Moon's Descendarium, which has good walkthroughs for all of the D3 levels.
P.S. If you feel like it, and you haven't done so already, try completing the secondary objective on this level, destroying the prison's computer systems. In medium security, there are two rooms full of destroyable data nodes; the doorways to these rooms are in the middle of Matcens. You'll then have to destroy the mainframe computer, which is located in maximum security. The objective isn't necessary to complete the level, but having an excuse to blow more stuff up is more than enough reason for me.