How do I view tags in an OPC server in OPC read block?

I am trying to read a tag value from a Compact Logix PLC in simulink using an OPC server. I have the OPC Toolbox. I am not able to view the offline or online tags when setting up OPC Read block in simulink. I used a conventional naming syntax for the tag to add it manually but I get the following error:

=== Simulation (Elapsed: 1 sec) ===

Error:Error evaluating registered method 'Start' of MATLAB S-Function 'opcslread' in 'PLC_Demo/OPC Read'. The following is the MATLAB call stack (file names and line numbers) that produced this error:

Caused by:

Could not create all items for read block 'PLC_Demo/OPC Read'.

One or more items could not be added:

Adding PLC311._xTest_AO1 failed with 'The ItemID is not in the server address space.'

Adding PLC311/_xTest_AO1 failed with 'The ItemID is not in the server address space.'

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I installed "OPC Foundation Core Components" as instructed in the Rockwell Automation forum. I tested the server using an OPC Test Client provided by Rockwell. It says that the OPC server is accessible and the tag value is being read successfully. How can I include the ItemID in the server address? Or better yet, how can I browse the existing tags in the PLC?