Thanks for pointing this out. I have updated the guide with a warning for now. Yes, it's pretty clear that something is wrong. The builds for DIR-615 are usually 3.4 MB. This one for Rev. C is 3.8 MB.FOLLOW AT YOUR OWN RISK! I'M NOT RESPONSIBLE IF YOU BRICK YOUR ROUTER!
Here's what to do:
1. Download: https://dl.dropbox.com/s/wcweio7b4tyee3d/board_config?dl=1
2. Download http://portableapps.com/apps/development/frhed_portable
This is a portable hex editor, meaning it will install to a folder you specify, no registry entry or other settings. When you're done with it, you can just delete the folder. Browse to your desire installation location when presented by the installer.
3. Open FrhedPortable and go File > Open and select the board_config you downloaded. Note that the doesn't and shouldn't have an extension. Just leave it as it is.
If you never used a hex editor before you might feel a bit overwhelmed by all the weird stuff. Don't worry. Scroll to the bottom.
4. You have two columns. At the end in the right (smaller) one you have something like "[...]yyyyyC1.y00:00:00:00:00:00yy00:00:00:00:00:00.yy0x10.yyyy[...]"
What interests us are the two 00:00:00:00:00:00 series. Everything else before, between and after MUST remain the same.
The first 00:00:00:00:00:00 needs to be replaced with the MAC address of your LAN/Wireless card, while the second with the one for you WAN card (port). These should be printed on a sticker on the back of your router.
Move the cursor over the first 0 in the first series and typing the corresponding MAC as it appears on the sticker, with : in between the 2-character pairs. Then move the cursor over the first 0 in the second series and do the same for the WAN MAC.
When done, go File > Save, and then close.
5. Upload the file somewhere online where you can get a direct download link from. You can also use SCP to get the file onto the router.
6. Flash the router from the emergency interface with the r16214 build from ftp://dd-wrt.com/others/eko/BrainSlayer-V24-preSP2/2011/02-17-11-r16214/dlink-dir615c1/dir615c1-factory-to-ddwrt-firmware.bin
6. Get Putty Portable http://portableapps.com/apps/internet/putty_portable. This is based on the same principle like the Hex editor. Install anywhere and delete when done.
7. Run Putty, put the IP address of your router, select Telnet and connect. Input username as "root" , regardless of what account you use for web interface. The password is the same as the one you use on the web interface.
8. Once connected type: cd /tmp/
9. You should now be in /tmp/ directory. Type: wget -O board_config http://replace.this.with.your.download.link.to.board_config
If you use SCP copy the file in /tmp/ and you don't need to use wget.
10. Type: dmesg|less
You should see something like:
[ 0.410000] Creating 8 MTD partitions on "ar7100-nor0":
[ 0.410000] 0x000000000000-0x000000030000 : "RedBoot"
[ 0.420000] 0x000000030000-0x0000003e0000 : "linux"
[ 0.430000] 0x0000000fe000-0x0000003b0000 : "rootfs"
[ 0.430000] mtd: partition "rootfs" must either start or end on erase block boundary or be smaller than an erase block -- forcin
[ 0.440000] mtd: partition "rootfs" set to be root filesystem
[ 0.450000] 0x0000003b0000-0x0000003e0000 : "ddwrt"
[ 0.460000] 0x0000003e0000-0x0000003f0000 : "nvram"
[ 0.460000] 0x0000003f0000-0x000000400000 : "FIS directory"
[ 0.470000] 0x0000003f0000-0x000000400000 : "board_config"
[ 0.480000] 0x000000000000-0x000000400000 : "fullflash"
Only count the entries with the long hexadecimal values like 0x0000003b0000-0x0000003e0000 and count with RedBoot being 0. This means linux=1, rootfs=2, etc., board_config=6.
11. (Optionally, you can do: dd if=/dev/mtd6ro of=board_config2 in order to backup your current board_config. Use SCP to copy the board_config2 off your router, because it will disappear on reboot.)
Otherwise, type: dd if=board_config of=/dev/mtd6 (replace the 6 after mtd if your board_config is something other than 6).
12. Go to interface and reboot router.
if the dd command does not fix the wireless, do it all again until that step and try: mtd -f write /tmp/board_config board_config
I know. Unfortunately this is not for the average user
If you don't understand something feel free to ask and I'll try to help.