The SANscreen support matrix should be your guide for discovering Brocade switches - we list what firmware versions are supported for which Brocade datasources.
As a general rule of thumb, if you are running Brocade firmware 5 or higher, you want to use the Brocade CLI datasource. This datasource uses SSH for Service Insight (inventory discovery), and uses SNMP for Application Insight (performance collection). If you are using either Virtual Fabrics or Admin Domains, SANscreen needs a true admin level user - we need visibility into all the VFs/ADs to discover the switch. Otherwise, a read only user account will work.
For McData, you have options - either the McData SNMP datasource (you will need one datasource per switch), or EFCM datasources (version 7 through 9.x - DCFM is not supported)
What firmware are you running?