How can I change MAC address in router ASUS DSL-N11? Firmware version 0.00.18
Check out Asus' support/download page for that router here.
If, once there, you go to the Downloads tab, and then pick "Others" as your OS, you will be offered Firmware and Manuals to download.
The first one on the list of Manuals is entitled "ASUS Router MAC Clone Setup Procedure":
Step 7 (page 4):
There should be a MAC Address field in that section: