It's normally root but if you've changed it.............Found elsewhere might be worth a try m8
If you happen to forget your TM root password, here's how to do it:
If you have an image that accepts plugins from internet
1) Install TuxCom plugin
2) Run it from Yellow button
3) Point to /var/etc/passwd
change the line containing root into root::0:0::/:/bin/sh
now you can telnet your TM with an empty root password and create a new password for root.