Open Terminal, "cd" to the directory of the folder. Then "cd" into the .app file and go to "Mac.app/Contents/MacOS". Once there type "chmod a+x Karlson". Once you're done, just type "./Karlson" and you should be good to go.
P.S.: You have to re-do this every time you open the game but not the "chmod" part, that only has to be done one.