What do you mean by "active"? To activate it you just turn it on in the settings, connect your USB cable, and use HTC Sync.
If you mean "active" as in over WiFi, then you would need to root your phone to do this, but it is possible. Only, just like on any phone that does wireless tethering, your phone will run out of power very quickly, and the solution to that is to keep it connected to your laptop via USB, and if you're doing that there's no need for WiFi tethering.
If you mean HTC Sync doesn't work on a Mac, then that's not the phone's fault. That's one of the best known biggest issues any Mac owner will have - lack of supported software compared to a PC.
However, you can do USB tethering on a Mac by using PdaNet: http://www.junefabrics.com/android/mac.php give it a try for free. If I remember correctly the only thing you would ever have to pay for it access to https sites. But you get a two week free trial of that access anyway.