Well, I've got my Developer Certificate linked with it's private key. Works Fine.
I've downloaded my iOS Profile of distributor, but it says Valid signing identity not found.
So I checked my certificates, and exactly, my developer certificate it's linked with its private key but not my distribution certificate. How can I link/add my private keys to this certificate!?
Will this solve the problem "Valid Signing identity not found"?
Yes, the error you are getting means that there is not a private key on your Mac associated with the distribution certificate you are trying to use to sign the app.
There are two possible solutions, depending on whether the computer who requested the distribution certificate is available or not.
If the computer who requested the distribution certificate is available (or there is a backup of the distribution assets somewhere)
If the computer where the distribution profile was created is not accessible anymore (and there is not a backup)
You have to revoke the certificate and create a new one.
You may need to ask your team admin or agent to give you some privileges in order to generate distribution certificates. Once you have enough privileges, follow these steps (accurate as of 15-May-2013):
Reopen Xcode and check your project configuration to see if you can now select an "iPhone Distribution" certificate (i.e. it's not grayed out).