This Java Server runs on Websphere Application Server 6.1 and I need to monitor the Java Virtual Machine's virtual memory. Thanks.
Take a look here. This will show you how to configure a JVM for SNMP.
What exactly do you want to monitor? If you want to monitor Java Virtual machine's different memory types (heap, non-heap etc.), you can also use JMX Monitor. If you are interested in monitoring virtual memory of the Java process, you can use process monitor (WMI) for this purpose.
This can be done using SNMP or JMX. Our Java Application Server (SNMP) application template includes this information out of the box. Alternatively you can monitor this information using JMX using the Find Processes, Services, and Performance Counters Wizard under SAM Settings.
consider JVM Monitor