I was facing same issue, resolved using command line “xcodebuild” tool script, which is preinstalled with Xcode 6 (didn’t need to re-install Xcode 5).
http://www.thecave.com/2014/09/16/using-xcodebuild-to-export-a-ipa-from-an-archive/
Script in terminal:
xcodebuild -exportArchive -archivePath $projectname.xcarchive -exportPath $projectname -exportFormat ipa -exportProvisioningProfile 'Provisioning Profile Name'