I just updated from 14 pro to 15 pro. I cannot open any demo projects at present as it says I have to activate my license. Where is my activation code stored? Please help.
You should use the Download Access Code and enter that code in the Steinberg Download Assistant (SDA). This Download Access Code is sent by mail and also shown after you purchased the license.
Furthermore you may need to manually activate the product (Cubase 15 Pro) in the Activation Manager.