Sounds like you're turning the mounting bolt for the tensioner. The bolt on the wheel where the belt makes contact is what you want to turn, and actually you want to pivot the wheel upward. This is what relieves the tension on the belt. The tensioner has a good deal of tension, so be careful when you let it back down. You'll want to pivot the tensioner counter clockwise.
As far as the DOHC goes - VIN #: S is the DOHC engine, and VIN#: U is the OHV engine. The VIN (vehicle ID Number) is on the driver side dash , and can be seen through the outside of the vehicle through the windshield. I'm not sure which location in the series of numbers the "U" or "S" will be, but within the series it will be a "U" or "S".
I hope this helps!