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
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.