If it's just a single menu command you can just use the standard way of doing it for an app on OS X: Open System Preferences, go to the Keyboard & Mouse preference pane, click on the Keyboard Shortcuts tab, click on the plus and fill out the form to add your shortcut (remember that Menu Title should be the exact title of the menu item, including any ellipses(…)).
Comments
If it's just a single menu command you can just use the standard way of doing it for an app on OS X: Open System Preferences, go to the Keyboard & Mouse preference pane, click on the Keyboard Shortcuts tab, click on the plus and fill out the form to add your shortcut (remember that Menu Title should be the exact title of the menu item, including any ellipses(…)).