Do you want to create a macro to run inside Visio? In that case, the easy way to go is to open your file, then go to Tools menu, and choose Macro > Record new macro. Do the operations you require and Visio will generate the VBA code for you, should look like:
If what you want is to control Visio from a VB6 app, you have to obtain a instance of the application and then run a similar code. I haven't done it with Visio, but with Excel and can provide sample code if required.