Press 'U' to open up the officer/ship interface thingy (the one where you assign where your skill points are spent, what weapons your ship is mounting etc.) and select your ship on the left hand side, and click the 'Stations' tab at the top, there it should have the options to assign bridge officers to your ship's specific stations.
No, my problem is (and apologies for not clarifying), is that I can assign officers to the other stations except for one. That last station is an Ensign Engineering station. I have a bridge officer candidate that is an Ensign Engineer, but I can't assign her despite the open spot. I have all other spots filled.
To note, it looks like this:
Ensign Tactical - filled
Lt. Engineering - filled
Ensign Science - filled
Ensign Engineering - open
Despite that I have two Ensign engineers ready. They say there are no bridge spots open, but as you can see from the list, I have a spot open.
When you say your ensign engineering officer is a candidate, do you mean he is on the list of officers on the left, along with your own self/captain and the ship, or is he on the list to the right?
If he's on the right list, you'll need to get that officer to join your crew before you can assign him. If he's on the left, sounds like a bug.