Have you run the the HTML Help Workshop on its own at least once on the machine? After installing, you have to run it to establish the windows registry entries required for Arbortext to find it.
Try closing Arbortext, running hhw.exe from the HTML Help Workshop directory, closing HHW, and restarting Arbortext.
If that doesn't work, try searching the PTC support site for information about it, there are some documents there that describe the exact registry entries needed for this to work.
--Clay
