There are two passwords.
The first is the username and password used to protect the modem's web interface from unauthorized users. There isn't a default username and password set, so if one has been set, it's been done by you, somebody else, or the ISP who owns the modem. See here for password reset instructions
The second is the EXPERT password, used for configuring advanced functions not available through the web interface. The EXPERT password is derived from the MAC address, and can be found by using this webpage.
The EXPERT password is entered when accessing the modem's telnet command line interface.