You should have a fair amount of tension, you'll have to play with it to see what your belts like, but the platen should bow the belt out a bit when you look at it from the side.
I'm working off memory here so bare with me.
Remove the belt. Where the platen attaches to the upright, there should be two bolts that when you loosen them, they should allow you to move the platen in and out and angle it. You'll just need to keep adjusting until it feels right.
There's also two knobs on the upright that will let you extend it to get more tension from the belt.