So it's an 816. The computer doesn't do much, since the Voice Mail is added from the system owner's box just as easily. Try #6 *999 and if you're lucky it has no password. If you're not, you need the owner's password to "add a mailbox". It's option 5 or 6 and follow the prompts. If you don't have the system passwords, then you may be able to use the serial port with hyperterm to get into the admin programming and reset the passwords for mailbox 998 and 999.
As far as adding an extension, the system uses the 8 jacks on the main cabinet and the 1st and 2nd pairs or first or 2nd pair of every 3 in the expansion card cable. If they are undisturbed, they are numbered 101-116. You can plug in 3 types of phones, an analog or digital Panasonic phone or a single line telephone. They will use 2 pairs the 2nd pair or the 1st pair respectively.
LkEErie