From the DD-WRT online manual:
If you want to use user/password to login using SSH use user "root" with the password you set in the webinterface
So, you cannot use admin, you must use user root. And that's besides having enabled sshd in the GUI, as already suggested by @Darius.
To be clear: I have control of 4 DD-WRT routers. On all of them, with the command
ssh IP_Address -p port_number
the connection is refused. The connection is established if and only if I use:
ssh root@IP_Address -p port_number
The reason is simple: if I do not provide the user name root, the system will try to log me in the router with the same username I am currently using, which, in my case, is just my first name. And, of course, such an account does not exist on any of my routers.
This being said, a connection also requires three conditions: that sshd has been enabled through the GUI, that the password provided is the same as that of the administrative account used to access the GUI, and that the port coincides with that chosen at the time of the enabling the sshd daemon.
There is, luckily, nothing else to do. And, by the way, change the authentication method to a cryptographic key ASAP.