This is a tough question... I assume you have tried the Technician, Admin, and Installer passwords? Remember that they may be different if the default extension or password length has been changed, it is not uncommon for a 6 digit password to be set by default, make sure you are using the proper default passwords for those lengths. Sometimes that mailbox becomes 99 or 9999
If this is truly Express Messenger and not embedded express messenger in an ICP, then defaulting the card is a bit of a chore, but I haven't done it in probably 10 years so I don't know if the software will even work on new machines, or if it is still available from Mitel.
Purchasing a "new" card is impossible, it's been discontinued for decade, although several refurb houses likely have one available.