Cannot download apps from Mac App Store

After I click "OK"

I am using Snow Leopard from an external harddrive on my MacBook Pro 7,1. I am very well aware that Snow Leopard does not receive security updates anymore, but I do need to run the OS because of the features (PowerPC apps,Dashcode…) it has. I don't want to dual boot into Mavericks all the time (for example when I am designing websites) while I could do the same in Snow Leopard.

The OS itself was transferred with a time machine to an external harddrive and other than that everything else runs perfectly. What should I do to be able to download from the Mac App Store?

Thank you