I was having this problem today and found that after I installed the "Fat Channel Vol 1" bundle and restarted, I then had to use the navigation from the Studio One program main page. Go to "Studio One" then in there to "Activate Purchased Items". This brings a pop up box with plugins and extensions for you to activate. You're meant to be able to click the big green "Activate" button, but this NEVER worked for me.
How I did get it to work was by loggin in to the PreSonus website, go to each individual plugin I own via "products" and then in this example "add ons", the "view more details". Do this for EVERY FAT Channel addon you have listed, then click "offline activation". Manually download each licence, correcly naming them on the way as they all download with the same name "PreSonus.licence", which is a bit shambolic.
Once you have downloaded all your licences, then drag and drop them into the "activate Purchased Items" window and they should now work.