You don't have to click on the next check-box at all. All you need do is exit the content control, by clicking anywhere else in the document.
I'm not sure why you'd want to do that, unless the idea is to allow the user to save the document, then come back to it later on. In that case, though, Shift-F5 should take you to do the last edit.

That said, adding a bookmark isn't at all difficult. For example, after:
If ContentControl.Type = wdContentControlCheckBox Then
in the 'ContentControlOnExit' macro, you could insert:
ActiveDocument.Bookmarks.Add Name:="LastUpdate", Range:=ContentControl.Range.Characters.Last.Next.N ext
Note that the same bookmark name is used in each case. Of course, you'd probably want to add some extra logic to test whether the exit state was different to the entry state. Just adding the above on its own will bookmark any checkbox you exit. Adding the logic would require modifying the ContentControlOnEnter macro to capture the entry state so it can be tested before doing the update.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]