Where do you have RB installed? Are you using Vista, and if you are did you install outside the Program Files directory tree?
Once you have that information, and assuming you are *not* installed in Program Files on Vista, find the RB directory and then copy the anrb.exe in it and save it as anrb.exe.old or similar.
Then, download the 3.11 anrb.exe from the first post in this thread and save it with that name in the same folder, when asked to overwrite the existing file select yes.
Restart RB and look for 3.11 beta on the splash screen before the window opens.