The tensioner has a spring loaded mechanism inside of it, coil the spring around make sure its hooked. Bolt it back down and make sure it covers the bottom of the spring to keep it from moving out of place anymore.
The tensioner on the Ford Ranger is auto adjusting. There is no manual adjustment. It is usually adjusted by a 1/2" or 3/8" drive rachet. Once it is released all the presure is applied. When you push agaisnt it - it releases the pressure. This is used to change the serpentine belt.