The following article will probably be useful:

http://help.remote-backup.com/index.php?/Knowledgebase/Article/View/370/0/how-do-i-back-up-folders-and-files-that-i-add-after-the-first-full-backup

If you have been doing backups of the old server, and you migrated all the data to the new server, and the RBackup Endpoint with it, using the same login and encryption key, backups will continue as before, backing up new and changed files. Since you have been backing up this server, you should still have its older backup files on your RBS Server.

This is the WRONG way to do it, and WILL CAUSE PROBLEMS WITH RESTORE: If you re-installed the RBackup Endpoint software on the new server and changed its encryption key, even if you used the same login name and password, the new server will NOT be able to restore the old files. It will continue backups, and any files backed up from now on will be able to be restored. Old files will show up on the restore interface but will fail to restore because the encryption key has changed and the RBackup software has been reinstalled. If you supply the old encryption key, you CAN restore the old files.

If this is a server that you have not been backing up, and this is its first backup using RBackup software, set RBackup as described in the article linked above. Use Modified Dater/Time or Fast Pick. This will back up all selected files as a full backup the first time, then it will switch to Incremental.