Following instructions from here: WZR-HP-G300NH Router: Firmware flash and brick recovery through TFTP and takinginto account this comment:

First: It won't work if you connect your Mac directly to the router, as per step 4 of the Windows instructions. You need to have a switch between the two (if your old wireless router still works, it will do). This is because the Mac uses carrier detect to automatically bring network interfaces up and down, and your ethernet interface needs to be up and running at the moment you power the router on, or you'll miss the little window when the tftp transfer can happen. So, ethernet cable from Mac to a switch, and ethernet cable from the switch to one of the ethernet ports on the router.

Second: before issuing the put command in tftp, you need to do "binary". Adding "trace" is nice as well, as you see progress as the transfer happens. So, your tftp session should look like this:

But noting this: DD-WRT Forum :: View topic - problems recoberying buffalo wzrhpg300nhv2
Buffalo WZR-HP-G300NH2 is totally different from WZR-HP-G300NH.

I ended up here: Buffalo WZR-HP-G300NH2 - OpenWrt Wiki and downloaded: http://dl.dropbox.com/u/38634684/openwrt/openwrt-ar71xx-generic-wzr-hp-g300nh2-squashfs-tftp.bin

I then set up Mac to working switch, switch to bricked router.

Router Not powered up

Set the computer's static IP to 192.168.11.2, netmask 255.255.255.0, gateway/DNS 192.168.11.1