(Beware Buzz-users; Buze supports PVST, but also implements its own native VST-wrapper which is described here.)
VST instruments rely on MIDI messages to play notes, and therefore depends on another plugin to generate MIDI notes.
For example, one could use another VST, such as a VST implementing a pianoroll to generate MIDI notes. Or, as we'll discuss here, use one of the built-in MIDI generators.
To see this in action, first create an instance of a VST plugin. Let's use Joachims TB4005 as an example.
Right click in the Machine View background and find "TB4005" from the context menu. Then right click in the Machine View background and find "MIDI Tracker" from the context menu. Then connect the MIDI Tracker to the TB4005. Notice how the color of the wire is blue, indicating a MIDI wire instead of the black audio wire.
Finally connect the TB4005 to the Master with an audio wire. Click on the MIDI Tracker plugin and play some notes on your keyboard to hear the TB4005.
Many Buzz plugins also support MIDI. It is possible to connect the MIDI Tracker to both the Infector as well as the TB4005 in order to make them play the same notes. Note however for Infector, you need to set up Infectors MIDI channel properties. To see these properties, right click the plugin in the Machine View and select "Machine Properties".
Consider using the more light weight Note Generator plugin instead of the MIDI Tracker.