Clockwork Mod Can't find backups
ClockworkMod needs to be updated to properly understand Android 4.2's new filesystem structure so it can see its previous backups.

Technically, everything stored on the sdcard is actually stored under /data/media, however Android uses bind to essentially make a really fancy shortcut to various other places. This includes /storage/emulated/ and /sdcard.

As for deleting, without knowing the errors you were running into, it limits my options on recommendations. But you can use adb to delete files (with the code below, replace "old-backup" with the folder name containing the backup you want to remove).

adb shell
su
rm -rf /data/media/0/clockworkmod/backup/old-backup