Do you put inside the tire?

Duct seal, is kind of like clay. You put it on the back side of the rim.

I personally use adhesive lead tape to balance my wheels. Stays on much better and is heaver by volume to other materials (quicker, easier, less messy, and stays on better)

http://flickr.com/search/?q=Balancin...21351974%40N04

(Also note the top flite balancer)

-Jeff