Clean-up greyed out on Manage Backup Destination

I want to merge (collapse) some of my incremental backups.  However, the Clean-Up button is greyed out.  The drive drop down has my external drive in it.  However, the backups are within a folder on the drive.  I believe the program doesn't see the backups on the root of the drive and therefore it keeps it greyed out.

 

How do I change the DRIVES dropdown to have the path: B:\BACKUPS as opposed to B:\ (as it shows now).