first, a short disclaimer - we, boxee, have not devised the method of loading applications onto the apple tv. you have the great guys at atvusb-creator to thank for that, and for making it relatively simple. and of course, those preceding them at finding the methods of doing it in the first place.
so one resource on how to really do it is of course heading to atvusb-creator and following instructions.
a possibly easier to follow guide is available at Wired.com, with a more in-depth version at their howto wiki.
two things to always keep in mind:
- the atvusb-creator will treat your external usb harddrive as a thumb drive, at least until the next version. make sure you are formatting the correct device.
- you can always go back, power-off and on while pressing menu+minus, select language, factory restore, and update your atv to latest version.