Or E4200: The default IP address is 192.168.1.1 and "admin" is the default username and password.
Kyawa would appear to have it right, and since the OP didn't note which version he has it might be easiest to just try both. I note that on older routers you could enter "admin" for the username or you could leave it blank. Entering the username as "admin" should work in both cases but it wouldn't hurt to try both options. If the username is optional it won't hurt, but if the username is required then there is only one choice.