We're running AppMon 6.5 and monitoring Oracle RAC. I assume you're referring to the note about limited RAC support in the 6.5 DB Agent?
"We currently offer limited support for RAC. You can either connect each of the RAC nodes separately to have a real visibility into the cluster OR connect the cluster as one single DB instance (but in that case the Database Agent will only monitor the node to which it is connected)."
@Jan-Hendrik: you need to specify the following, for the button to become available:
Once I complete these field, the button becomes available. Note that for RAC, you connect to the VIP/RAC IP or hostname, after which the nodes are discovered automatically. Each node will consume licenses, so if you run two nodes each with 8 cores, you'd need 2 DB agent licenses (2 x 8 CPU cores).