Since this is often a source of confusion for new users, let's start by explaining that the Translation Memories view is not where you add or enable a TM to be used with a project or file in Studio.
What You Need to Do
To add your TM and be able to use it, first go to Project Settings. To make sure you add the TM to the right project/files, make sure you have double-clicked on your project in the Projects view to activate it. If you're working with a single-file workflow, save it in the Editor view and Studio will automatically create a project for you.
Then either right-click on the project or go to the Project menu, and choose Project Settings.
With Translation Memory and Automated Translation active, go to the right-hand panel and either Add a TM or Create a new one. Make sure the Enabled and Update checkboxes are checked. Any TMs you add here will be used in your project. No need to use only one, you can use as many as you like.
After you click OK, when you're in the Editor view with one of your project files open, the Translation Results window will show the TMs active for that project.
By going to Project Settings again, you can add or remove TMs from a project at any time.