The first thing I would do is remove the belt and make sure the machine turns freely by hand. It may need a good oiling. With the belt off check the inside of it and see if it is glazed at all, which might cause it to slip. You can use some sandpaper to rough up the inside of the belt so it grips a little better.
Regarding the tension you do not want it as tight as it will go. Get the motor in the correct position on the mounting rail and with the bolt as tight as it will go but where it will still allow the motor to slide up and down, lower it until there's tension, tighten it up, and test it. You can run fabric under the presser foot when testing.
If you need a new belt I try to buy parts from Central Michigan Supply. I would not buy the rubber O-ring belts, even though they are cheaper. You can search this board for information on sizing a belt, I think there were some posts on it not too long ago.