Hello Kechagioglou and welcome to the forum.
The AmpliTube 4 download contains an installer for 3 software products:
-- the AmpliTube 4 standalone program and plug-ins,
-- the Custom Shop standalone program,
-- the Authorization Manager standalone program.
Run the installer and install all three. You may need to update the Authorization Manager, from here:
https://www.ikmultimedia.com/products/authorizationmanager/Run the Authorization Manager and Register your AT4 Serial Number (make sure you you the one from AT4 in your MY Products Area, not the one for the (free) "AT4 Custom Shop").
Run the Custom Shop, login, go to the AmpliTube section and click [Gear} >> "Restore my Gear". Then start the AT4 standalone program and check that all the gear models are listed.
If you run into problems, then let us know exactly what you have done so far and what problem(s) you ran into.