Yes, I agree, it can be a bit frustrating if you do not know how and if this is your first try. I assume from your writing that you have joined QMC@Home project for example by creating an account there. If not than let me know and I will tell you how. Otherwise, continue and do the following:
1. In your Boinc Account Manager select the QMC@Home project and go to the projects Home Page by clicking on "Home Page" in the menu on the left.
2. In the Home Page Menu that appears from the menu on the left, select "View and Modify your Account" by clicking on it.
3. Sign In by completing the User Name and Password Options.
4. In your account page select from the menu on the right "Find a Team" by clicking on it. (If you are already a member of another team than "Quit Team" first than click Find a team after that)
5. In the Screen "Find a Team" enter in the search field: SETI.USA and click "Search"
6. In the screen "Search Results" click on the team Name SETI.USA on the left.
7. In this Team Description Page Click "Join this Team" which is about the 5th option under the SETI.USA Statistics logo.
Good luck and thank you for your help. Let me know if it does not work or how far you got before you got into trouble. Enjoy...