Just to clarify you have a Two node cluster (Active-Passive) with MS SQL Server 2012 running on the Cluster? If that's true and you want to add another SQL Instance you wouldn't choose Add Node. That's used for adding another server to the cluster which would make it a three node cluster.

To add another SQL instance you run Setup from your SQL Server 2012 media, and under Installation choose "New SQL Server failover cluster installation". You need to have storage available on the cluster for this new resource and a network name and IP Address set aside for the instance. Also you need to decide if you're going to run Active-Passive or Active-Active, which this might be dictated by your licensing.