I had the same problem when I tried to use the PST II. The Durametric software didn't work for me either. It wasn't until I gave the instructions below to the dealer and convinced them to give it a try did it finally work. There is something with the '03 and '04's not having the Order Type programed in the DME.
http://www.renntech.org/forums/index.php?/topic/9917-obc-enable/
For Boxsters with the new instrument cluster, if there is no option in the Instrument Cluster Coding for “with On-board Computer” use the following procedure:
Do a Control Module Search from the Vehicle Type screen.
Go to DME control unit.
Go to Vehicle Data.
Scroll down to Order Type.
Press the F7 key to edit.
Enter either 986310 for Boxster (2.7) or 986320 for Boxster S (3.2). (Use up/down/left/right arrow keys). Press F7 to confirm.
Go back to the Vehicle Type screen and do a full Control Module Search. (Essential).
Go to Instrument Cluster, go to Coding and recode for “with On-board Computer” and press the F8 key to save.