If you have already installed DD-WRT on your Linksys WRT54GL you can still use UseMyNet but you must flash the unit with the UseMyNet (OpenWRT) firmware. Any settings you have made on DD-WRT will be lost.

Open the DD-WRT control panel

Click on "Administration" > "Firmware Management"

Click "Choose File" and select the UseMyNet firmware file you have purchased

Click "Upgrade"

Then follow the normal setup instructions found here:

http://usemynet.biz/dl/UMNPreinstalledSetup1-9.pdf