I purchased the Professional +, for up to 3 computers, but can't figure out how to add on the 2 additional computers to the main computer.
Moved this to ccleaner discussion board. And pointed the developers to this thread.
@jeppari after you install CCleaner (https://download.ccleaner.com/ccsetup556.exe) on your other PCs the serial key code that you got when you purchased online should work to activate those as well (Menu Options > About > Upgrade to Pro).
Same with the additional software that comes with the bundle - Defraggler (https://www.ccleaner.com/defraggler/download/standard), Recuva (https://www.ccleaner.com/recuva/download/standard) and Speccy (https://www.ccleaner.com/speccy/download/standard)