Go to your collection and click on the box for Might and Magic 6-pack. Below the box should the downloads for the installer (about 728MB) and for the extras (about 518MB) appear. Just click on the installer to download it. After the download is completed go to the folder where it is stored and double click on installer. Follow the instructions of the installer and everything should be fine.

I clicked on the installer button (thanks for the reference), but this is the message I am getting:

The address wasn't understood

Firefox doesn't know how to open this address, because the protocol (gogdownloader) isn't associated with any program.

What do I do now?

It seems you don't have the GOG downloader installed, but try to use it. You should either install it or deactivate it (click on the button above the extras and switch it to "off"). You don't have to use it, but can still download through your normal browser.

Wish I could upload screenshots from this computer, it would make the explanation much more easy.