Carl,
I'm sure adding items to a collection is not supposed to be this finicky, but this has worked for me:
1. Make sure both the "search" and "add/modify" boxes are checked for catalog(s) you plan to use.
2. Create and name a collection in the collection tab.
3. Click on the collection and make sure it is highlighted.
4. Click in the search box, type in the desired search terms, and hit return to execute the search.
5. Select the images in the contact sheet that you wish to add to the collection.
6. Right-click (or command-click) in the gray space BELOW the last collection name in the list. From the pop-up menu, select "Add selected items here."
7. The images should be added to the selected collection and the collection's count should increase.
Although this works, I think it would be better if we could right-click on the name of the collection and select "Add selected items here" rather than right-clicking the gray space beneath the last collection in the list.
Marv
Mac Pro Quad Core 2.93 (2009/Nehalem)
20GB RAM
OS X 10.10.5