If you have a Schrader valve (fat round valve like cars use), then you need a valve core tool like this.



Apparently there are several different models. The business end is those two prongs at the top right. Anyway, snag one at your local auto parts store, or department store. Remove the core, and put the slime in. If it still doesn't go in, fish around with a toothpick or spoke to see if you feel an obstruction.

If you have a presta valve. Look for two flat spots at the end of the valve. If you have those, you're in luck, snag the valve core off with a small crescent wrench. Wrap the electrical tape around the valve stem, and squirt through the hose and into the valve.

If you have the presta valve, but don't have the two flat spots, then watch the video I linked above.