If you installed it correctly, it should be located under "Modules", "Shipping". Double click on the "free shipping" module, then click "Install". You can then turn the module on and off, as well as set the other options.
The problem I'm having is that it only works if the customer buys a product from the free shipping categories only. If they add a product that is not in the free category list, it charges shipping for both items. I've tried every possible configuration but nothing gets it to work properly. It either charges shipping for both, or calculates free shipping for both, depending on the options selected in the admin configuration. This seems like an easy fix, but unfortunately I'm not a hard coder, and am hoping someone picks up on this, as it would be a very useful contribution.