If you're getting the error:
urlsnarf: br-lan: No such device exists (SIOCGIFHWADDR: No such device)
Fear not it's an easy fix. You just need to swap out br-lan in the module code for eth0. Here's a foolproof guide on how to do that:
1) Load up your LAN Turtle, then 'Esc' at the main menu to take you to the Turtle terminal.
2) Type the following to go to modules
3) Use the following command to edit the urlsnarf module
4) Hit Ctrl + \ and type
hit enter then type
hit enter again and then press A to apply the replacement to all instances of 'br-lan' in the code
5) Hit Ctrl + O to save the module
6) You're done! Ctrl + X to exit.
urlsnarf should now successfully read any non-secure web-traffic...remember HTTPS site traffic will not show here!