I made it to the observation deck. This is what happens :

1st time : Activate teleporter, somah comes through, looks at me for a few seconds, goes back through the teleporter. No one else comes through. I go to the engineering core and find somah standing by the inaccesible door to the bridge teleporter, sally and everyone else is missing.

Reload.

2nd time, sally comes through and unlocks the door. I run into the room with the observation window, and turn around to find EVERYONE who was following me has dissapeared. The talking alien head does not appear.

Reload.

3rd time : The talking head appears after sally crawls through the duct. Finshes talking. At this point Paulson dissapears(i dont know where he went), medic guy and somah are now running into the wall next to the door sally was supposed to unlock, and sally is not unlocking the door.

Reload.

4th time, somah comes through the teleporter, sally comes through, immediately goes back through to the engineering core, i follow her to find medic guy and sally standing at the inaccessible door to the bridge teleporter. Paulson is nowhere to be seen.

Please tell me what i have to do to get to the bridge without cheating.

5th time : Same thing as 4th time.

6th time : Sally unlocks the first door, everyone but sally follows me into the observation window room. I look away for a split second and sally dissapears. Medic guy and somah start running into the wall and i find sally standing at the inaccesible door to the bridge in engineering core. Paulson dissapears too.

7th time : Sally unlocks the first door and immediately runs back to engineering core followed by Paulson.

Unless you just get lucky, that's usually the way the buggy scenario plays out. Use the console to unlock the second door and get on with the game. They'll catch up with you later.

I went through the door to the weapon's lab and eventualyl found all of them standing around next to 3 teleporters in the...biological or experimental lab. Whats up with that?

Zeta is just one big bugfest. The quest randomly completes the wrong stage and sends the group to the bridge too early. One time I had to use the console to unlock the doors because the gang kept teleporting back to the wrong level so I went on without them. They loaded in next to those three teleporters and immediately died with no enemies around! Delamer made a fix for the quest bugging out http://www.fallout3nexus.com/downloads/file.php?id=12291

you know I keep hearing that Zeta is buggy.. to be honest I never had a problem with it. never.
actually, all my add-ons are bug free.