Hi ,
when you click on Additional folder check-box it will open a folder hierarchy to choose any folder in your system, from which you want to load Plugin in PSE from next launch onwards.
Suppose I have plugins on C >> xyz >> abc
I can navigate to C to xyz to abc and choose it to load from next time.
There is also choose button to change the folder hierarchy
In PSE Plugins nomenclature is uses I have never heard Add ons for PSE.