This is a cut and paste from the guides that I used. I have omitted some parts, edited, and added others. I take no credit for the process its self, just for putting it together for the HSMM-MESH users. Credit goes to the DD-WRT Wiki, and the HSMM-MESH Firmware installation guide.

1. Download the CFE_Updater-WRT54G-RG.bin from here: ftp://dd-wrt.com/others/tornado/CFE_Updater/
2. For the RG Router double check that you got the file with RG in the filename!
3. Download the TFTP.exe file from here: ftp://dd-wrt.com/others/tornado/Windows-TFTP
4. Download the svn 12548 NEWD_Mini.bin (NOT NEWD2!) file from here: ftp://dd-wrt.com/others/eko/V24_TNG/svn12548/
5. Read the peacock announcement here: http://www.dd-wrt.com/phpBB2/viewtopic.php?t=51486
6. Do a proper hard reset on your router, according to the instructions at note 1 of the peacock announcement. Because you do not have dd-wrt firmware on the router at this point, you will not be prompted for a password.
7. Connect your computer to your router with a lan cable, connecting to a LAN port on the router.
8. Set your computer to a static IP address of 192.168.0.17 and a 255.255.255.0 subnet mask.
9. Open your browser window to 192.168.0.1. You should see the linksys webgui. If you don't power cycle your router.
10. Go to administration, firmware upgrade
11. Select the file CFE_Updater-WRT54G-RG.bin (the one you downloaded in the first step) and click upgrade.
12. GO FOR A WALK for at least 5 minutes. IF YOU TOUCH YOUR ROUTER YOU WILL LIKELY BRICK IT!
13. Change your computer to a static IP of 192.168.1.17. THIS IS VERY IMPORTANT!
14. Open a command window and try to ping 192.168.1.1. with the "ping 192.168.1.1. -t" command. You should get some timeouts. When you get a ttl=100, wait two more minutes. Keep this window open.
15. Open up tftp.exe
16. Select the address of 192.168.1.1. Leave the password blank. Set it to 99 retries and pick the file dd-wrt.v24-12548_NEWD_mini.bin that you previously downloaded.
17. Hit upgrade.
18. GO FOR ANOTHER WALK. WAIT AT LEAST 5 MINUTES. Watch the ping window. You should see ttl=64. When you do, wait two more minutes.
19. Set your web browser to 192.168.1.1. You should see the dd-wrt password page.
20. Power cycle your router.
21. Navigate to http://hsmm-mesh.org/download/firmware/index.html and download the appropriate firmware for your node hardware.
22. Set your browser to 192.168.1.1, and navigate to Administration, Firmware Upgrade.
23. click Browse, select the hsmm-mesh firmware you downloaded.
24. Click Upgrade.
25. The first time mesh firmware is installed on a router it will take about two minutes for the first reboot to complete.
26. The installation is now complete, you can now proceed with the node setup.

I hope this helps.

Tried to follow instructions, got my tongue caught in the ceiling fan.

Seriously, thanks for the effort and posting the info here.

I wish more folks would contribute like this.

The method is solid.  WX5U, where are you getting lost?

Just joking.  I assume the procedure works as listed.