Client Error Messages on Mac
Game cannot be opened
Error Message: "The game cannot be opened"
On Mac OSX you can work around this by entering the below into the Terminal.
xattr -r -d com.apple.quarantine /Applications/RuneScape.app/
To open the Terminal on OSX, open your Applications folder > Utilities folder > "Terminal".
You can also try running the downloaded executable directly, or running it from the .dmg file.
Black screen when loading
Try using the JAVA Client instead.