First get yourself a tool like Spy++ (comes with MS Visual Studio, let me know if you need a download) to find out the particular IDs in question. In general, (basically) all GUI elements are windows. To influence the behaviour of a certain control (button, checkbox, etc.), you just have to know the window handle and what message you want to send to it. You can find a list of common messages in a particular header file (let me know if you need it). This way, you can just (for example) send the button the message that the user pressed the mouse button while the mouse cursor was within the limits of the button. The application does usually not pay attention to the source of the messages it receives.
I'm not too familiar with VB (I'm rather a C++ fan *g*), but I'll see if I can find some example source.

If you want to do it completely programmatically, these are the steps:

a) Find the top-level window of the application in question
b) enumerate the child windows until you find the appropriate one
c) send it the message you want

I can make up some VC++/MFC sample code if you like. You will be able to find most API calls as VB definitions on the net, so that shouldn't be much of a problem...