Mambo / Joomla passwords cannot be recovered as they are set using a one-way hash function (MD5). However they can be reset to new values.

The user information is stored in the MySQL database (obviously) in the mos_users (for MAMBO) or jos_users (for JOOMLA). The prefix (mos or jos) can be configured and may be named differently for your system.

Using phpMyAdmin
In phpMyAdmin select the database for your Mambo / Joomla system on the left combo-box. Then choose the mos_users (or as appropriate) table and select browse. Click on edit icon (pencil) for the row where the username is admin.

In the password row under Function column set the function to MD5. Set the Value to your chosen password. Save it (by clicking on Go) and you are done.

Using MySQL commandline or GUI
With MySQL you can execute the following command in mysql command line or gui:
UPDATE mos_users SET password = MD5( 'Your Password' ) WHERE username = 'admin';
Note: Change Your Password to your password (obviously).

And you are done!