What are the login for SDM? Why does it need 2 level login?

I recently enabled a ssh access to a local machine. Which means there is a nat to forward port 22 to an internal pc and acl enable port 22 for dialer interface.

After that, when I tried to log into sdm, i was asked to provide ssh user/pass. I tried to use the same one, but failed.

But I still can telnet into the router. And there is no nat translation happening when I tried to login through sdm.

Can anybody explain this to me?

Thanks,