Put the extension tube in the other end of the main tube. It should slide all the way in. You didn't remove the brg off the extension tube I hope???
If it slides all the way in. Put the clamp on (with the knob) and tighten the two screws, and put a piece of duct tape over the end. Putting on the saw head makes things to arkward at this point. The extention tude should move freely all the way in.
Just to make sure, the extension tube was NOT disassembled Correct?