start the bolt with the spring relaxed. Tighten it to the point that you can just barely pry the spring into place with a pry bar. Hold it there, and then tighten the bolt some more.
The way I've done it the last few times is to loop the old timing belt over the tensioner (with the smooth side against the roller) and use that to pull the tensioner over and then tighten down the bolt. I always have a hard time finding a good place to pry off of in there.