I have downloaded a provisioning profile from my id, but when I drag into Xcode, and then in the provisioning profile section it says
a valid signing identity matching this profile could not found in key chain
I am trying to download the existing profile and drag into Xcode, but this gives me the problem that how could I deploy my application to the device?
Delete all your private and public keys. Create one new one by using the assistant to request a certificate...
Before you do anything else name the key by double clicking on it and typign a name that identifies it as the one key you will keep.
Delete all others.
See this question which you might have found with a quick search, from earlier today.