Make sure that the driver is installed correctly. The tutorial linked by AndyFC above lists all the required steps. Because P96 does not support the PiStorm out of the box, some manual actions are required after the installation.
The most important part is the file emu68-vc4.card in Libs:Picasso96. Make sure it is there and not protected against reading.
The second important part is the file in Devs:Monitors. You can name it like you want, but in the tooltypes there must be BOARDTYPE=emu68-vc4. That's the name of the .card file without .card. Make sure it is spelled correctly, that there is only one BOARDTYPE= tooltype and that it does not have brackets around it. Also make sure that there is only one P96 monitor file.
