I've noticed that a number of people have found my site via Google looking for a way to add the check numbers field in the Quicken Essentials for Mac registry. I was initially befuddled by this issue, too, when I didn't see any place for check numbers. And yet, I could select a past transaction that had a check number and the inspector told me the number was there. Despite Quicken's parent company name (Intuit), I found that adding a check number field was not very intuitive at all. Nevertheless, I found it.
If you want to add the check number field, go to your rightmost column, which is probably unnamed. Right click on the blank title space to reveal a list of columns that can be included in the visible registry. Choose Number and your check numbers will now display in your registry.
Note also that the columns can be rearranged in any order you want simply by dragging their title field at the top. This is certainly a nice improvement over the previous version.
UPDATE MARCH 20, 2010: This week, Intuit released an update, Quicken Essentials version 1.3f4519, which adds a Columns command to the View menu, bringing the program better in alignment with standard Mac interface standards.