The variables you create in PAC control are not directly available via Modbus, only items in the memory map are. In your strategy, you could transfer the variables to the "scratchpad" which is located in the memory map, and then they will be available through Modbus. If you already have a lot of invested Modbus code in your PC project or you HAVE to use Modbus, this may be the simplest way to get going.
Also, check out the REST API: http://developer.opto22.com/rest/pac/ You can get IO points and variables using it.
There is also a .NET Controller SDK that can read variables http://developer.opto22.com/dotnet/