It doesn't work on the aria...
I think they completely removed the vanilla launcher fron the aria.
Like it was said before, you can just install another home app. My personl favorite is SlideScreen, which shows any new voicemail, missed calls, sms messages, calendar events, your google reader feed, twitter feed, stocks, and facebook messages (note, I am not part of the SlideScreen team, I've just found it to be really great). I've also tried out HelixLauncher, which I like more than LauncherPro, Sense, or the vanilla launcher.
You should know that the HTC widgets only work with the sense launcher, so if you're looking for a nice flip clock widget you should get Beautiful Widgets, which has a copy of most of HTC's nice widgets.