You can find some ActiveX component that does the Modbus issues for your it works as both OPC server and client. For example check http://www.automatedsolutions.com/
You can install Kepware's KEPServerEX which has the Modbus Ethernet or serial driver to talk to the PM500 and we aslo provide VB6 and VB.net examples using the OPC Foundations OPC DA 2.0 Automation Wrapper. For more advanced .Net support use the server and our ClientAce .Net Design Time Package.