Hi Tia,
The reason you cannot filter the Test Plan page by Release is because Test Cases and Test Folders are not schedulable into timeboxes so that value does not exist in their filter options. You have to do this action via Test Sets, which you schedule into an iteration, release or milestone and assign Test Cases to them. A Test Case can live in multiple Test Sets across multiple timeboxes. You can see Test Sets on the Iteration Status page, Release Status page or Work Views. You can then filter Test Sets by release.
I think these options will give you what you are looking for. Hope this helps!