Xcode 4 - "Valid signing identity not found" error on provisioning profiles on a new Macintosh install

With Xcode 4.2 and later versions, including Xcode 4.6, there is a better way to migrate your entire developer profile to a new machine. On your existing machine, launch Xcode and do this:

1. Open the Organizer (Shift-Command-2).
2. Select the Devices tab.
3. Choose Developer Profile in the upper-left corner under LIBRARY, which may be under the heading library or under a heading called TEAMS.
4. Choose Export near the bottom left side ofthe window. Xcode asks you to choose a file name and password.

Edit for Xcode 4.4:

With Xcode 4.4, at step 3 choose Provisioning Profiles under LIBRARY. Then select your provisioning profiles either with the mouse or Command-A.

Also, Apple is making improvements in the way they manage this aspect of Xcode, and some users have reported that the Refresh button in the lower-right corner does the trick. So try clicking Refresh first, and if that doesn't help, do the export/import sequence.

Picture for Xcode 4.6 added by WP

Edit for Xcode 5.0 or newer:

1. Open Xcode -> Preferences ('Command' + ',')
2. Select the Apple ID from the list.
3. Click on the SETTING icon near the bottom-left corner of window, and choose EXPORT ACCOUNTS... Xcode asks you to choose a file name and password.

On your new machine, launch Xcode and import the profile you exported above. Works like a charm.

Picture for Xcode 5.0 added by Ankur

I just run into the similar issue today. Unfortunately my HD died on me so I couldn't do the migration mentioned here in the accepted answer. I had to do the following steps:

1. Connect to the Apple Developer member center then the iOSprovisional portal.
2. Revoke my certificate.
3. Create a new certificate by providing a new pair of private and public key.
4. Remove all the previous provisioning profiles and create new ones.
5. Download the new provisioning profiles and install them in Xcode by just draggingthem to the Xcode icon in the dock.

The same action is also mentioned on this post.

Make sure your certificate is in the "login" keychain. Highlight the login keychain if you don't see it, search for it. Then drag the cert over the words "login". Close and re-open Xcode, ta-da.