In the same vein as some of the other sites out there, you are able to search for the songs/artists you want to add, and the simple and smooth interface will do the rest. Once you enter a search (it even prompts you for things you may be trying to type) it lists all the tracks in can find that match. A simple click of the ittle plus sign next to it and that track is added.
Fully worth checking out, as they have a pretty diverse range of music. A lot of underground emcees show up on there, as well as random indie rock bands and anything else you can walk. And you can even preview it before you add it to your list.
So head over to MixTurtle and give it a try. If you love it, sign up and you can save your lists. For now, this seems to work outside the US (unlike Pandora), so it may be useful.