Once you have installed an addon from the store, you can immediately start using it in your tests. Open the test recorder and hover over an element and freeze it to bring up the element inspector menu. On the actions menu you can search for the addon you want, and with one click you can add it to your test.
Depending on which addon you are using, you will need to fill in different input parameters in order to use the addon. Each addon should have labeled fields that let you know what you need to do in order to properly use it. Fill out those fields and click on the Create button to add a step to your test that uses the addon you've selected.