Buffalo, unfortunately, encrypts their firmware, and their routers will accept only encrypted firmware in the web interface.

WHR-G300N is now only supported by dd-wrt V24-preSP2, created by BrainSlayer .

The first time you flash, the TFTP method can be used to bypass this ‘encrypted’ firmware requirement. Afterwards, the router firmware can be changed through the Web interface normally.

TFTP flashing

The latest DD-WRT build has a new, easy, and safe way to flash a WHR-G300N router. To do this, just Tftp_flash the firmware.tftp image to 192.168.11.1 in the 1st three seconds of booting when the router is in recovery mode.

The trick with this router is to wait for the blinking red light to stop and then start the tftp transfer. Be ready to press enter the very moment the red diag light stops blinking.
UPDATE:

There is a more convenient way to flash the firmware:

Every time the router boots it looks for a file called “firmware.ram” on a tftp server at 192.168.11.2. Now if you start a tftp server (e.g. TFTP32) and offer this file (simply rename firmware.tftp), the router fetches it automatically and flashes itself. This way you don’t have to try to hit enter at the right time and it’s fully automatic.