To confirm, you've been able to successfully connect to the PLC by creating a channel, adding a device, and adding tags in NI OPC Servers? You just want to be able to monitor the tag data?
You can check the tag data by opening the OPC Quick Client that installs with NI OPC Servers. There's a button to do this from the NI OPC Servers Configuration window, or you can open Quick Client from your start menu by going to Start->All Programs->National Instruments->OPC Servers 2012->OPC Quick Client. In Quick Client, you'll be able to select the channel, device, and tags you've added and monitor their values in real time.
The process of using Quick Client is also outlined in step 4 in the section titled "View Existing PLC Tags with NI OPC Servers 2012" in the document I've linked below.
Connect LabVIEW to Any PLC Using OPC
http://www.ni.com/white-paper/7450/en/
Regards,