Another easier way of opening a project’s .mkb file with Xcode:
Just search for the “Marmalade” folder with spotlight. It is usually located in “/Developer/Marmalade”.
Now go to the 6.X/Applications folder (5.x, or whatever marmalade version you have installed).
Double click on the “Mkb” application found inside. It will complain because no .mkb file was supplied. Whatever. Press enter and close the window.
Curiously enough, from now on, you can open Marmalade projects by double-clicking the .mkb file, instead of calling mkb within a terminal 🙂