On the fallout wiki it says you need to get into the overseers office to access the pulse gun, in the office i found a very hard computer, is this the computer that opens the armory doors? I left the vault becaurse of radiaton poisoning.

No , but you need Blackwell's key unless you have 100 lockpick.

It's a great little gun for robots and enemies who wear power armor.

Nope, you don't need 100 science to get the pulse gun. getting through the vault doesn't require any skill checks, but the skill check you found is a shortcut to get there faster. Once you get to the pulse gun you either use a key which is not located anywhere near the vault or you use 100 lockpick skill.
http://fallout.wikia.com/wiki/Pulse_gun_(Fallout:_New_Vegas)

What's the puslegun?

http://fallout.wikia.com/wiki/Pulse_gun_%28Fallout:_New_Vegas%29 its basically death incarnate for robots.

So, there is a terminal in the overseers office? And its not the one that is very hard?

There is, and it's [VERY HARD]. But, you can always find the password for it by exploring around V34, and open the Armory door.
Once inside the Armory, you would need to either lockpick the [VERY HARD] footlocker, or use a certain key that you get elsewere.
Check the wiki page, as others have suggested already, for all the gory details.
Cheers!