This might be completely wrong, but I think it has to do with the time you have the clip playing. If the clip isn't long enough, you can't put in certain transitions. E.g. You can't put a transition in that takes 1.5 seconds from each clip if they are only 1 second each.
As I said, I could be wrong, but I think I have had that problem before.
as the previous poster mentioned you can just drag the transition in between. if you want to use "add", select 2 clips by using "CMD-Click". Then, the "add" will be enabled. you can then use that to add the transition. either one should work though.