Well, you can get general up/down alerting, CPU/Mem statistics and (if you have NTA) netflow collection without custom pollers. If you want something to alert on outside of the SNMP trap viewer rules, then you'll have to establish some custom pollers and set them up for alerting, although in my experience with Riverbeds, most of the important details regarding these systems is reported via SNMP traps, so you'll want the trapping set up as well.
In either event, if you want to set up some custom pollers, the MIB starts as '126.96.36.199.4.1.17163' . Go to your server and set up some UnDP's. From there, choose what you want, add 'em to the appropriate nodes, and so on so forth. Then you can add the Custom Poller Status property to your Node Details page and it should populate the current values related to whatever pollers you've created.
Out of boredom/the realization that I had no UnDP's set up for my Riverbeds, I went ahead and made this UnDP package: Riverbed UnDPs.UnDP. If you have Steelhead Riverbeds, then you can use these as a starter.