The simplest "Visual Interface" is to use the Visual BASIC built-in graphical controls (buttons, text boxes etc) or to use the draw functions to draw lines, circles or shape. Most basic Visual Basic text book will have very clear explanations on how to use these.
When you click on a control you may want something to be sent to the PLC. This is the communication portion. You will need to study Chapter 3, 4 and 5 of the T100MD+ User's manual for the communication protocol that the PLCs support.
If you want to use only the native protocol, then we have made available a sample Visual Basic program that you can download from:
http://www.tri-plc.com/applications/VBsample.htmYou only need to call the SendCommand function and provide the command string that you wish to be sent to the PLC and the function and handle the rest and return you a response string from the PLC. It is very easy.