Thats correct, you need the Canon conversion tube and the DCA. The DCA will come with a set of rings to couple both together.
The only limitation to this setup (and most others) is in bright sunlight with the sun falling on the LCD screen, you may have difficulty seeing the screen.
As Neil mentioned if after putting the DCA on the scope eyepiece you find any movement between the them, get some tennis grip tape (it is non sticky) and wind around the eyepiece. (We both use the grip tape)
Don't forget to remove the zoom rubber eyepiece or otherwise the DCA may not go all the way on
Bob
This is a link to more of my shots taken on the Canon A590IS
http://www.flickr.com/photos/jingbar/sets/72157609602990824/