I\'ve been trying to enable another developer on my team to build an iPad app that we are creating for a client, but am having no luck getting his development certificate upload
Note that only certain types of Certificates can be added to certain types of Provisioning Profiles.
For example, an "App Store Distribution" provisioning profile will only show you the option to add "iOS Distribution" certificates. All of your other certificates will not be shown in the Provisioning Profile editing interface.
I hit an issue similar to this question, when one developer held all of the Distribution certs and another held the Development certs. I thought there was an issue adding the certificates to a provisioning profile due to the multi-user scenario.
The actual issue was in matching the correct types of certificates to the given type of provisioning profile.
You need to invite the developer to your account in the apple developer site.
Member Center > People > Invitations > Invite a Person to Your Team
This should automatically install their developer profile and then you can sign the app with the "Team Provisioning Profile".
Also check out Organizer > Provisioning Profiles > Automatic Device Provisioning
in Xcode 4; this simplifies installing profiles.
You can add additional developers only if you have signed up as a company. Ref: https://developer.apple.com/support/account/