Nothing happens when I click the “Add a tab” button.
Most of the time, this is caused by a plugin/theme conflict.
Before anything else, you can try deactivating all of your plugins (except Responsive Tabs) to see if it solves the issue. If it does, re-activate them one by one to find the one that's conflicting. You can also try switching to a default WordPress theme if deactivating plugins didn't solve your issue.