There are a number of posts on plesks website here is one:
Why dont they support thier product on freebsd machines?
If anyone here can help with the question please respond as I myself have attempted to upgrade php on a freebsd plesk machine with no success. On the redhat systems its like a peice of cake, but on freebsd it seems as if the plesk people just threw it all together kinda in a proprietary manner where if you upgrade something you break the software / scripts that make up plesk.
I like plesk, but if I can't get something figured out, I will be moving to cpanel, as all of my cpanel boxes work like a champ regardless of OS redhat or BSD.