When you have your project open in Xcode there's a section called Code Signing. Click the first drop-down menu and you should see a list of identities, you should see at least one iPhone Developer and iPhone Distribution identity. If you don't an iPhone Distribution identity, or it is under the "identities without Provisioning profiles" subheading, you can resolve this in iOS Dev Center.
Go to iOS Dev Center > iOS Provisioning Portal > Provisioning > Distribution. You could have an expired profile, if so just modify and re-save it. If there is no profile then click New Profile. Then you should see the new iPhone Distribution identity in Xcode which you can now choose.