If this is a reinstall of OneCare, first configure backup on this PC to perform a backup to the external drive from this PC. Run the backup, then perform the custom restore and you should see the other backup(s) to select from for the restore.
If this does not work, please contact support for help.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator