Is there a shortcut key to comment multiple lines in matlab editor

The comment shortcut is CTRL+R on Windows and CTRL+/ on Unix systems (not sure about OS X). To comment multiple lines you'd have to highlight them using the mouse or SHIFT.

In any case this can be customized via Preferences -> Keyboard -> Shortcuts.

In Mac OS, Command + R to comment code.
Then, Shift+Command+R to un-comment.