Add to dashlaunch's launch.ini as a plugin, e.g. plugin1 = Hdd:\xbdm.xex
And then FTP it to the location you specified.
Once that's done, to take screenshots, install the Xbox 360 SDK (can be found in this forum section), and go to Xbox 360 Neighborhood in Windows Explorer and follow the directions to add your 360 to the neighborhood.
Then all you need to do is right click the 360 icon and select take screen capture.
Things to not do:
If a dvars type is String and its value is empty, don't edit it.