Ok, so... I attempted to load Tomato on my RT-N53 yesterday afternoon. I first loaded the DD-WRT "initial flashing" image, then restarted.
That is where my problems started. When the router would come back up, all led's (except power) would be flashing in sync 2-3x per second. Setting my laptop's wired lan to 192.168.1.10, I couldn't connect to the router (192.168.1.1). My network adapter kept showing that the "Network Cable Unplugged". Doing 30/30/30 restarts didn't help, as it would just go right back to all led's flashing again. I was getting pretty frustrated after an hour or so of this, so decided to leave it and come back later.
I came back a few hours later, to the sight of a router with no flashing led's and the power led on steady. I plugged a network cable back into my laptop and tried to connect to 192.168.1.1 and to my surprise, the DD-WRT page came up asking for a password. I was then able to log in without any problem! I changed the password in DD-WRT, then went to firmware upgrade and picked the Tomato firmware.
After the Tomato firmware loaded, I tried to log in using the password I set on DD-WRT, no success. I tried what BrianB tried (blank username, "admin" as password), no success. After trying multiple combinations, I looked up what the default tomato password is. Apparently, I didn't read the instructions
right, as I didn't know about this part:
If you're upgrading from the DD-WRT firmware, telnet into the router *before* upgrading and type "nvram get http_passwd". The result will be your password in Tomato. This is necessary because of a change in DD-WRT's way of using the standard http_passwd variable.
So, since I hadn't done that, I held the reset button down for a bit (20-30 seconds) and tried to log back into the router. This time, it came up with the Asus Recovery Mode screen looking for a firmware file. I browsed out to the Tomato firmware again and had it flash the firmware a second time. This time, it came back up asking for a password. The combination that worked for me was "admin" for the username and "admin" for the password.
Finally, I was logged into Tomato! Since I was going to be using the RT-N53 as an access point, I then configured it to have a static IP on the Basic tab under LAN and turned off DHCP. I also disabled the WAN, then finally set up my wifi networks. Finally, my RT-N53 is working as a dual-band access point!