Hi there,
I downloaded Krita for Mac a few days ago but I can't open the dmg file. I don't know what is wrong because the operative system is updated and I have the last version of Krita.
I know that this software can be unstable in mac, but I will be grateful if someone can give me a solution.

Which version did you download exactly? Can you check the sha1sum by going to files.kde.org/krita/osx or files.kde.org/3/osx (depending on whether it was krita 2 or krita 3), press the Details link for the right file and compare it to the sha1sum of the dmg you downloaded?

Because, chances are big that you just got a corrupted download. I test all dmg's on a different mac from the one where I make them, with a lower version of OSX, and they all just open.