As said previously:
1) delete the save game for the previous XBMP (just go into the Xbox dash)
2) You can add a manual line to your evox.ini file or the xml/ini file for whatever dash you use. I imagine autoadd is not working because you have either FTP'd it to the wrong directory or the XBMP directory is now more than 1 folder deep before you get to the default.xbe file which is what autoadd is looking for.
Example:
Your ini file autoadds everything in F:/Apps/. It goes to Apps and then into each sub-folder for xbe files - like F:/Apps/XBMP/default.xbe would be perfect. It is very common for people to actually transfer an extra folder over and the real path becomes F:/Apps/XBMP/XBMP/default.xbe - obviously the names of the folders are specific to you but regardless, generic autoadds often won't detect it 2 layers deep. Retransfer it over appropriately or add a line to your ini manually.