How to exchange the DD-WRT firmware on your router by OpenWRT.
my Router: TL-WR1043ND Version (DE)v1.0
Old firmware: DD-WRT r19519
New firmware: OpenWRT "Attitude Adjustment" 12.09 Beta 2, filename: openwrt-ar71xx-generic-tl-wr1043nd-v1-squashfs-factory.bin
First revert back to the original TP-Link firmware. Use the Web flash interface and a special image. This took very long in my case, first the 200sec in the browser counted down, and then some minutes passed where nothing happened. Finally the Browser showed a confirmation dialog reading "Update failed", but the router was unresponsive (no ping, no dhcp). I did a powercycle on it, then it was up and running fine, with the original firmware reading:
3.11.5 Build 100427 Rel.61427n Hardware Version: WR1043N v1 00000000
Then I simply put the openwrt /factory/ file from above into the Web flash formular and uploaded. Took some time, then OpenWRT was up and the LuCI Web interface asked to set a root password.
 got this instruction from http://wiki.openwrt.org/doc/howto/generic.flashing#via.original.firmware  from here http://www.dd-wrt.com/phpBB2/viewtopic.php?t=85237