itch.io

Open finder (that app with the half white and half blue face)  and click on applications, drag the logo  (The white starclan sign that you get on that popup window ur talking about) into the applications tab and it'll download. 

If you need a password from an administrator, just enter your logon password for your computer. If you yourself do not own the computer and share it with someone who does, you will need them to enter their logon password for this.