You can switch between full screen and windowed mode by hitting Alt + Enter (as usually in every app out there), and can also use the Cogwheel in the top right corner and Toggle between Fullscreen/Borderless/Windowed mode.
As for your packs, you need to enter your username and password on the Login screen. After completing the Tutorial for the first time, you can press Login in the Main menu and then you can enter your details, also click Remember Me to save your credentials and auto-login next time you open the software. Meantime I checked our backend and it shows you signed in successfully a few minutes ago, so I guess everything is sorted by now?
Please let me know if you face any other issues.