Hello there,
I'm also experiencing the same problem with autopilot - it doesnt work. The only thing that works for me are the vertical modes, but autothrottle doesnt respond. This means after leveling off at certain level, engines remain in full climb power (or derated according to FMS) resulting in overspeed. Lateral navigation (LNAV, HDG SEL, HDG HOLD) doesnt work at all.
IRS alligned, FD on, AT on, CMD A,B,C (tried all) and FMS fully programmed.
I also asked my friend - real 757 pilot for help and he also couldn't find out what's wrong. Maybe at this point someone will tell me that this is not a real simulation but just a game. Well that's correct but other addons which I own (another company, LVLD) are ok and don't have such stupid problem with autopilot.
One more thing. I tried to load CS757 after loading my default flight, and also after loading default flight you get in fresh copy of FSX installation.
And the last problem I get - autobreak is always in RTO position after loading CS757 and clicking on it (LMB and also RMB) doesn't move the switch.
This is my first post in this forum and I'm very sad it has to be like this. If anybody has got some suggestions I would be very pleased to try it. Thank you very much for responses.
Take care.

P.S. Why CS remains so silent??? They have to know about this issue, because so maaaaaaaaany people are experiencing it.