I recently had the opportunity to learn a lot about SNMPVarBind values and populating those values in the Alert description Field from a monitor and rule. A customer was having difficulty determining the proper syntax to use for this and with the help of some folks in the OpsMgr product group I've documented it below:

For a Rule:

Added "Simple Network Management Protocol" through Add/Remove programs on the OpsMgr server
Configured the community name as "public" and to accept packets from any host
In the OpsMgr console, ran the Network Devices Discovery Wizard
It discovered my SNMP device and I selected it to manage
Created a new SNMP Trap (Alert) rule and tested generating alerts using trapgen
Used the following syntax for the Alert description: $Data/EventData/DataItem/SnmpVarBinds/SnmpVarBind[1]/Value$
Tested by creating an SNMP Trap using trapgen.exe
Alert appeared and the alert description contained the value specified

For a Monitor:

Use the following syntax for the Alert description: $Data/Context/SnmpVarBinds/SnmpVarBind[1]/Value$

To figure out the proper value we used a collection rule and viewed the raw event data from the collection rule. Also, we found that if you enter more than 10 parameters for Alert description they won't get reported properly. This is currently a limitation in OpsMgr 07.