Driver/Kext Realtek 8191SE - Mac OS X Lion 10.7.4
Maybe the kext is for 32 bit mode only and does not work when running in 64 bit mode. Boot your Mac in 32 bit mode and check whether is works then. (If your Mac uses the 64-bit kernel by default, you can start up with the 32-bit kernel by holding the 3 and 2 keys during startup.)