If it downloaded correctly it should be in your appdrawer (default position of the button for that is in bottomright corner of your home screen, the blue button with 4 white squares in it), then scroll through the pages left or right to find your application, if it's not there, it's probably not downloaded correctly. I have found a couple of Market issues with my Ace, sometimes it doesn't start downloading properly and I have to hit the download button again, in the top status bar you can see if an application is being downloaded, finished or installed.