Sewing Machine Tension Knob Won't Turn?

I just got my sewing machine. It is a Singer Seventy. I am very inexperienced with sewing and am just learning. I just filled my bobbin and threaded my machine. Everything was fine until I got to the part of setting my thread tension. Right now the dial is set on 8. I need it to be on auto, but the dial won't turn at all. I read in my manual that the foot needs to be down. I put it down and it still won't move. I've tried turning it off, but still no luck. Please help!

By Sarah c