Its a separate device. nothing to do with your router. Consider it a small PC. Forget DD WRT, you can install proper linux i.e. Arch or Debian on it. You can install SAMBA, transmission, pyload. anything you want. Because it has a 1.2Ghz processor and 256mb of RAM, its much more powerful than these routers.
As for ordering, I had gotten it from ICC World (i2c international) or whatever they are called now. That time it was 35$ and the quote was 2.5k IIRC. you can get it via SNS as well(jr.com does accept international cards) but the box is a bit large so more shipping charges and I don't know how customs will treat it. iccworld goves you a fixed quote so i went with that.