Zenbeats supports VST and AU instruments as well as FX plugins. To use external plugins in Zenbeats go to Menu > Settings > Plugins and set your plugin directories by clicking one of the four blank slots. The most common VST plugin paths for Windows are C:/Program Files/VstPlugins and C:/Program/Files/Steinberg/Vstplugins.
Check these locations outside of Zenbeats before you set your plugin directories to make sure you are setting the correct path. After you have set your VST plugin paths, press the "Rescan All" button. It may take a few minutes to scan all your VST plugins.