Oh! I know how to fix this!
So those little tabs on the top are the different types of bricks, right? And you click on one to view it's contents. If you have a lot of add-ons, then you might have a lot of tabs, perhaps so many that you can't actually see the tab of the section you want. This is your problem
To fix it, simply click on a tab and press the 'tab' key on your keyboard. This will make it so you have selected the next tab after the one you had clicked before.
So click on the last tab you can see, and click the 'tab' key. You will be taken to the next tab, the one that, in this case, you cannot see right now. Now you can view it's contents. Then click 'tab' again, and you will go on to the next one etc.
Hope this helps!
~BlockDuck