The additional Libraries Folder is located wherever you want to place it, mine is in Documents under a B4A folder. This folder is where you place all your downloaded libraries from the forum.
You need to create a new folder in Documents (or just My Documents) and name it something logical like 'Additional_Libraries'. Now run B4A, click on the 'Tools' menu and go down to 'Configure Paths' and select it. You will now see a text box call 'Additional Libraries', click on the button to the right hand side and navigate to your newly created 'Additional_Libraries' folder, select the 'Additional_Libraries' folder and then click on OK.
Enjoy...