You don't. Just open up mYTube, use the search tool to find videos on YouTube and then you can watch them or download for later.

If you just watch a video, it will cache it and you can either save it or delete it off your device/MMC to save space or you can just download it and then watch it later.

It's pretty straight forward. Too bad you can't download the HQ videos. You always get the SD quality ones.

Jesse~