hi, prism54.ko should be under 'drivers/net/wireless/prism54/' folder under the src tree.
i guess it is reading the starting directory wrong? did it give u the right path when it asked about the path to the kernel and libs? (sorry, dont remember exactly, as i had done this qiuite a long time ago ...)
and im sure u have gone through this (?):
http://prism54.org/cgi-bin/cvsweb/pr...EADME?rev=HEAD