No Mac developer certificates found

I'm publishing a Mac application but at step 2 "Platform Specific Settings" the check box "Publish to Mac App Store" is gray with "No Mac developer certificates found".
I've created, downloaded and installed in Keychain my developer certificate "mac_app_identity.cer", how can GS found/install my certificate?
Thanks.
I've created, downloaded and installed in Keychain my developer certificate "mac_app_identity.cer", how can GS found/install my certificate?
Thanks.
Comments
3rd Party Mac Developer Application: Your Name
3rd Party Mac Developer Installer: Your Name
edit: have you looked in your keychain?
Thanks again.