I would like to copy to the tmp directory on my router, and do not know how to enable SSH to access it.
I am connected to it via LAN and am running OSX Snow Leopard 10.5.6.
Have you tried to search for the official documentation of DD-WRT ?
Here is the topic about SSH and telnet : http://www.dd-wrt.com/wiki/index.php/Telnet/SSH_and_the_Command_Line
This explains how to setup the SSH daemon and authentication methods.
The following wiki article explains how to enable SSH and access your router from the internet : http://www.dd-wrt.com/wiki/index.php/SSH_access_from_internet
Basically, here are the steps :
Now you will be able to connect to your router's WAN IP from the internet using an SSH client such as PuTTY.
Disabling Remote Access
If at any time you wish to disable SSH access from the WAN, - Goto the Administration tab and the Management sub-tab on the Web Interface - Disable "SSH Management" under the section titled "Remote Access" - Apply Settings
Hope this helps !