Yesterday Apple released iTunes 11, which brings an entirely new look to iTunes.
The changes are dramatic, but under the hood iTunes remains largely the same. This means that Wooden Brain Concepts software that interacts with iTunes – iDupe, MoveTunes, Cast Away, and OmniGrowl – are expected to work as they have with previous version of iTunes. As WBC had no advance access to iTunes 11, full functionality is now under testing. Updates will be made as necessary over the coming weeks.
For the time being, some words regarding iDupe specifically are in order. Since iDupe relies on using list checkboxes to deselect duplicate tracks, you must do two things. First, in the General tab of iTunes' Preferences, you must enable the option to "show list checkboxes." This was the case with iTunes 10 as well. Second, you must use a view that includes these list checkboxes. With iTunes 11, there is more than one way to do this.
If you want a view of your whole library, make sure that "Music" is selected in the upper left corner. Then on the top bar, select "Songs" and sort in a way that makes sense to you. Usually this would be by Artist or Album or Album by Artist. Just click the top header of the column to adjust the sort.
Alternately, you can select "Playlists" on the top bar. This is really the preferred way to use iDupe. Select your playlist on the left. Then in the top right corner, use the "View" button to select "List" and then set your sort as above.
In both cases, if your preferred sort column does not appear, go to the "View" menu item in the top menu bar and select "View Options…" There you'll be able to select the view(s) you need.
In either of these views, you can select the tracks for iDupe to scan as desired.
More info will follow as necessary.