I'm assuming you input credentials and that the credentials are showing as working. I don't have any WAN321's deployed out there, but I have a number of other devices at several sites that the backup is working perfectly.
Understood. Please see the Supported Devices and Feature Summary located here.
The WAP321 is not currently listed as supported so the backup functionality is not currently available.
They do have several links, including this one from April 2012:
That indicate that even though it is not in the supported devices, that they do support some functionality on the device.
Understood, and there is "some" functionality but it appears that it more relates to Wireless reporting than anything else. If the device doesn't show up in the Supported Devices and Feature Summary link I referenced above, then odds are it won't support config backup. That said, if it's only letting you enter SNMP, then it definitely won't support config backup as telnet/enable credentials are required for the ON100 to get a copy of the config.