Somewhat of a late response, but better late than never I guess--- perhaps this will help the next guy that comes along.
It sounds like your system is a bit different. You should check avaya.com, they have good manuals and documentation for all the versions.
For testing, use extension 10 or 11, as those are the 'administrator extensions'. On my system, extensions start with 10, if yours are different then I'm not sure what the equivalent is. Picking up an extension and hitting Feature-09-xx should turn on the MWI for extension xx, and hitting Feature-10-xx should turn it off. Plugging a normal phone into that extension port and hitting #-09-xx / #-10-xx should do the same thing. Note that this must be done using an *extension* port, not a line port. So you'd need asterisk to have another FXO card, plugged into an Avaya extension.
You don't need any voicemail hardware for the Avaya. Our setup no longer makes any use of the PartnerMail module, and we don't have the voicemail PCMCIA card. I would have removed the ParnerMail module (leaving the avaya of itself without any VM ability) but I had nowhere else to put it :)
Hope you got it all sorted out...