The communication interface is being provided by the program DSData.exe. It would be started on the PC before the VB program. The LinkTopic specifies the communication channel. The LinkItem specifies the precise item within the PLC.
The actual communication product is not available now from AutomationDirect.
There should be a 'help' file on the PC, DSData.hlp, which will provide some information on the setup.
The owners of this site, Host Engineering, are the developers of DSData. Go to the main site and look up information there.