Installing primary mower deck belt

Is there any trick to moving the tension arm to get the belt on? I use a friends garden tractor ( gray with white wheels, 50" deck) to mow the yard every two weeks, when the belt slips off, I found out that I have to engage the p.t.o. at low r.p.m.. I doubt it has been greased in a long time, I replaced the belt last year and it shows signs of slipping. it is a pain to get it on again because there is no place to put a 3/8" socket wrench like on our cub cadet 1550. I have to use a long pry bar and a c clamp to move the arm. There are a few notches in the arm, it looks like you have to put a wrench of some kind in them to turn the arm, what tools were there are gone so I have use what I have.
Gene