OPC Clients use two different methods to find and to browse the variable information of the OPC Server. Finding the OPC Servers installed on the PC can be done in two ways.
Reading out the Registry
Browsing with the additional program OPCEnum.exe
The additional program OPCEnum.exe is not distributed. So it can happen, that an OPC Client cannot find the control system OPC Server, because it uses this browsing method. Read the documentation of the OPC Client to find out whether the browsing method can be changed to reading out the registry (In the Matrikon OPC Explorer you will find it under Options). If this is not the case, you have to install the program OPCEnum. Hint: If you install the freely available Matrikon OPC Simulator Server, OPCEnum is also installed.
When browsing the control system OPC Server offers all variables that exist in the running control system Runtime. So you have to make sure, that the control system Runtime is running, before you start browsing the variables.