how do I asjust the serpentine belt on a 2005 ford freestar?

It should not require any adjustment. the right tension is provided by the idler pully. If you want to release the tension for serpentine removal or examination release the tension on the idler pulley in the rear of the motor, spin the nut in the center like you were tightening, and the pressure will be released from the serpentine belt. Reversely, the appropriate tension will be given using the same way when the strap is re-installed.

Almost all serpentine belts have a spring loaded tensioner to keep the belt tight. If you are having problems with squealing or slipping you probably need to see about having the belt replaced.