Well, it really is nothing else than doing this:

Please go to Your Account (Bookmark the place), then go to Change Password.
Type in a password you like, with a minimum of 6 characters, then type it again.
Then press the Change Password button at the bottom.

Wait a minute or 2.

Open Boinc Manager, Projects, Attach to Project, type in http://setiathome.berkeley.edu in the first window's box.
Press Next.
Select Use Existing Account.
Type in your email address (same as which you used to register your account with)
Type in the newly set password.
Press Next.

Or follow the Checklist:
1. You made sure Boinc.exe and Boincmgr.exe are allowed to talk to eachother on ports 1043 or 31416 if needed?
2. You made sure Boinc.exe is allowed to talk to the internet on port 80 and 1043 or 31416 if needed?
3. You went to Change your password twice, and used the Change password button there to save it? (You did not use Enter?)
4. You start up Boinc Manager, then go to Projects, Attach to project?
5. In the window that comes up, you fill in http://setiathome.berkeley.edu in the URL link and hit Next?
6. You then choose the (round) radio button in front of Existing Account?
7. You fill in the email address you used to register your account here at Seti?
8. You fill in the password that you just set and hit Next?

Go to your acct make a new password, open Boinc enter url, and use existing acct and enter password and email address.