WR743ND support was included in r28271 and at this time (2012/01/31) is not included in the stable branch. Currently I've some problem with trunk and wep encryption so I back-ported WR743ND support in the stable branch.
svn co svn://svn.openwrt.org/openwrt/branches/backfire
patch -p0 < tl743wd.patch
make menuconfig
To use only the external antenna on tl-wr741nd put
option 'txantenna' '1' option 'rxantenna' '1'
in the wifi-device section in /etc/config/wireless