I'm sure there is a way to do it with a mac, I just dont know how. If you're not able to find a method you may want to consider going to a local computer repair shop. There is a place down the street from my house that charges $45 to root a android phone and $50 to jailbreak iphones. I'm sure that you can find a place not too far that will un-root your evo and return it to stock. (its cheaper than bricking your phone and paying $100 to asurian for a refurbished one.
also, I did a quick google search for you and found this
The only way to UNroot an EVO on MAC. - xda-developers
Hope this helps.