I don't know if the server version requires anything special or not. Here are the steps I use for 10.3.x Panther.
Download the client and place it in its own directory (folder).
(Check out mikkyo's optimized version 3.20 under the resources thread)
Open a terminal window
CD to the directory.
chmod +x boinc_3.20_(whatever you named it)
./boinc_3.20(whatever you named it)
It should start running and ask you to enter the URL for the project you want, and your "ID key".
If you are lucky, SETI will actually be up and running and you will be able to attach to the project and download work. As of last night and tonight, SETI-BOINC has been down most of the time.
If SETI isn't up, and you want to keep the CPU busy until it is, try attaching to
Predictor@Home until SETI is available. You can set a low resource share for predictor (like 5) and then it will only get work once in a while or when SETI isn't available. Note: If you attach to Predictor, and then later attach to SETI, you'll have to wait until the predictor units are completed before any SETI units will download. Don't worry though, it won't take long on a G5 server.
If that doesn't work, post what messages you get in the terminal.