XBMCProjectM should be checked out into the sources/visualisations dir at the same level as XBMC (as it is in svn).

projectM now uses cmake, so get it:
sudo apt-get install cmake

For complete projectM build instructions see:
http://projectm.wiki.sourceforge.net/Installation+Instructions

Unset CC and CXX environment variables or you'll upset CMake:
unset CC
unset CXX

Quick and dirty projectM build instructions:
make
