I guess you won't be able to monitor MySQL over SNMP. You need to use the "ODBC User Experience Monitor". Your basic metrics of the box will be polled by snmp and your database metrics will be pulled over ODBC.
Here's a link that explains it Monitor MySQL with SAM - Setup/configuration steps
Hope that helps
...or make an ssh connection and use command-line scripting and/or the MySQL template in SAM.