This site explain step by step that what you need to do Certificates, Identifiers & Profiles and as your question
"Valid Signing identity not found"?
You need the private key that were used to sign the code base with provisioning profile. . If you don't have then you can generate a new signing request on the iOS developer portal.
For Export:
Xcode -> Organizer, select your team. Click Export. Specify a filename and a password, and click Save.`
For Import:
Xcode -> Organizer, select your team. Click Import. Select the file containing your code signing assets. Enter the password for the file, and click Open.