First, without the belt on, make sure that you can rotate the motor up and down the full length of the curved slot. If it hangs up partway down, that is your problem. Clean up any burrs or obstructions that limit that movement. If that cannot be done easily, return the saw.
Assuming that the first step went well, run the blade all the way up. Pull the motor back so that you only have about 1/4" play before the spring in the motor mount plate bottoms out (fully compressed). The spring is designed so as not to overtension the belt even when almost fully compressed. Tighten the slide bolts. You do not want the motor mount bolts to bottom out when the blade is all the way raised so that there is no movement. That can cause overtension on the bolt and bearings. As you lower the blade, the spring tension gets less, but that is compensated for by the weight of the motor.
Practicing at practical wood working