Problems with re-installing former CS5 Premium license from Adobe account

Hi - Peter from Brussels calling.

I have the following situation. Have a official copy (still have the serial number label) of Design Premium CS5. This was on one of my old (2012) Macs running when I needed it (not much) ... InDesign, Illustrator, Photoshop.

Now this Mac is dead. I found another older Mac on which I could re-install it. I cannot use my BU, since the OS of that Dead Mac is different from the OS of the replacement Mac.

But I need desperately to be abble to open the files made with this CS5 version (documents saved in the cloud)

I logged-in to my adobe account to download the original Adobe license (a +3Gb file).

The download went well .. But in the next fase (installing) ... It gets stuck with a message : "Install can't be opened. You should move it to trash" .. Can Anyone Help me with a sollution?

Thanks !