I had problems installing Parallels Tools in Backtrack 5 R1. After spending too much time searching for a solution i finally found an embarrassingly simple one. And since sharing is caring, here you go.

Before you install Parallels tools you need to:
root@bt:~# ln -s /usr/src/linux /lib/modules/
After that the installation ran just fine.

Relevant specs:
Parallels 7
Os X 10.7 (Lion)
Backtrack 5 R1