It's actually a little tricky to explain how to activate and manipulate HEAT. I think the best way is in the mix window...
1. Open your mix window
2. In your mix window view selector make sure HEAT is checked (that will give you controls for each channel)
3. Then in the bottom right corner of the mix window is an arrow that will open the master HEAT controls. (It's exactly opposite of the track view arrow on the left side.) Opening this will give you the master on/off button for HEAT as well as the Drive and Tone Controls.
I'm sure you probably have this, but may have missed reading it. Here is a link to HEAT option guide that has a detailed step by step description of how to use it!
http://akmedia.digidesign.com/suppor..._v10_67826.pdf