Found some pics from my interior trim refinishing project, which might help you understand a bit better what you're dealing with here. I remember pulling up the front of the lower seat bolster. Then you can expose the hidden clips which reach forward along the seat. I can't remember exactly how I got them to release, but recommend exposing them so you can see what you're dealing with.
I'm sure there are sources on the internet on how this work, if you search. 996 Porsche uses the same seats and same trim pieces, so this is not Boxster 986-specific, it's shared by 986 and 996 platform. To get the front seat bottom to release, there are two plastic clips under the front edge of the seat that have one pin each that needs to be pulled out, before they will release and free up the seat cushion. That's where I think you'll want to start. No need to remove seats from car.
Edit: just looked at dghii's post. You'll want to perform step 7 - removing the white plastic clips from underneath the the front of lower bolster. No need to remove seat from car to do this. Then, you can work the seat covering up high enough to access the trip piece's foreword fastener. Pry out the lock pin, then pry out the remaining cylindrical tabbed piece. Be careful not to overstress, snap, or crack the plastic. Good luck!
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