The following should work:
Make sure the "Apply to" field points to the tab in which you expect the script to run.
The "current tab", as shown above, is the visible tab (the active one) in the most recently focused window.
You can use the "Run" button shown in the screenshot to test the script in a specific tab.
If you are sure that the script is running in the correct tab, then maybe the problem is that you are running it too early, before the page is loaded.
If you are using an event trigger such as Tab load begins, you should change it to Tab load ends so that the script runs when the page content is fully loaded.
If you are still unable to make it work, please post a screenshot of what you are doing so we can further assist.