Quoted from : http://barryezl.posterous.com/compiling-and-submitting-in-xcode-4 (thanks!)

"In the menu, select Product > Edit Scheme
Change the Build Configuration to Release
Now make sure the correct code signing identity is set. Start by clicking on the project icon at the root of the Project navigator.
Choose your main app target (i.e., the project name under Targets)
Click Build Settings
Click All because by default Code Signing Identity is hidden (*facepalm*)
Make sure a valid distribution profile is set under Code Signing Identity > Release
In the menu, select Product > Build For > Archiving. Make sure there are no warnings about code signing.
In the menu, select Product > Archive (step #9 may be redundant, I'll have to test)
Open the Organizer. The new archive should be found under the Archives tab.
Select the archive and click the Submit... button.
Postscript: You can save some fiddling by making a dedicated Release scheme. Go to Product > Manage Schemes, click + to duplicate the default scheme and set it's build configuration to Release. You'll find the new scheme in the project pulldown menu."