I haven't used this feature much in my testing, but:
Look in /var/airwave/device-backup/ap/<ap_id>/
You should only see folders for ap_ids for controllers and switches, not APs
In these folders, there should be flashbackup files of type tar.gz
Side note: I'm only seeing flashbackups tarballs for Aruba controllers/switches on my lab setup
What type of devices are you trying to download archived configs for?
I'd be interested in knowing if the non-Aruba devices also create tarballs which I might try later on.