Are you talking about the Ariba Live app? If so, then that is meant to assist attendees of AribaLive and yes, you would obviously want/need to register for AribaLive. If you are talking about the "Ariba Mobile" solution, it is a mobile app that allows you to approve requisitions. That app isn't supported in the upstream solutions, Sourcing/Contracts/SIPM. It is only for the downstream, P2P solution. Your company will have to have enabled its use in their P2P realm.
David