I had this issue as well, it would not recognize my Evo even with following the instructions down to a T. I also found that my computer was SLOW as HELL with Doubletwist running.
So I uninstalled and went with ItunesAgent to sync with Itunes and it was a breeze. No problems for me so far.
In case you are using Mac google ItunesMyWalkman, which is basically the same thing as ItunesAgent. Hope that helps