Right, do you mean the MASSIVE round door, in the "do not enter" bit?
If so, there's a control room on both sides. Go to the one on the left (as you look at the door) by pinging a few useful portals around. Then put one on the wall of the control room and the other all the way over by the other control room.
Hit the button, jump through the portal quickly and hit the other button.
Unless that's a different bit, in which case you know all this already.