I just added the USB cable from my IPFire device to my existing Cyber Power CP1500PFCLCD. And here are the changes I made to apcupsd.conf:
[root@ipfire ~] # cat /etc/apcupsd/apcupsd.conf
...
UPSNAME Cyberpwr # <- only 8 character allowed
UPSCABLE usb
UPSTYPE usb
# leave DEVICE blank
DEVICE
...
These are the only changes I made to the default apcupsd.conf file. And it seems to work OK for me. I have not tested this yet (no test button and no test power down).
EDIT: Here is the screen I see in menu IPFire > APC-UPS status
