Hi Elise- as you're seeing already, using simple filters requires that you start entering the tag name and then choose a tag (only one) from the result list and then you'll get a list of items with just that exact tag. In Plan >> Backlog you can only use filters but there's an alternative that's at least as simple as using a Custom List, and it has the advantage of letting you query more than one item type (e.g. user story AND defect AND portfolio item).
If you go to Plan >> Work Views then you'll find that if you add a Tags filter there and type ball into the tag name field then a pick list of all tags containing ball will present, and you can simply set the checkboxes next to all of the desired tags and get your results. If you haven't experimented with Work Views yet then there's a good help topic at Work Views | CA Agile Central Help . My users love them!