When there are errors causing “The application does not have a valid signature” when trying to drag an .app to the Xcode Organizer, it’s generally caused by expired or invalid certificates. The following was suggested by ingemar in another thread to clean up the Keychain.
Start the Keychain Access app, and select “All Items” (found under “Category” on the left).
In the search field (upper right) type: iPhone
This will filter out all iPhone related keys.
You should see your iPhone Developer and iPhone Distribution certificates there and also see in which keychain they’re in (Keychain column on the far right). You only need Developer/Distribution certificates in the “login” keychain.
Delete any Developer/Distribution certificates you see in any other keychains.
Also delete any expired Developer/Distribution certificates you may see (ones with a little red icon).