Check out Seti at home help desk at
http://setiathome.berkeley.edu/forum_help_desk.php. The unix/linux section has quite a few details, hints and tips.
In short, the directions are quite straightforward.
Download boinc_5.4.9_i686-pc-linux-gnu.sh from boinc.berkeley.edu.
You have to do the following commands via a command line. Open a terminal and run
cd Desktop
./boinc_5.4.9_i686-pc-linux-gnu.sh
If it doesn't run (smth like permissions denied error), run
chmod +x boinc_5.4.9_i686-pc-linux-gnu.sh
./boinc_5.4.9_i686-pc-linux-gnu.sh
It will print the instructions on screen.
Just in case, the following instructions are something like
cd BOINC
./run_client &
./run_manager &