Python modules which implement a Tk-based GUI are registered in the VMD "Extensions" menu by calling VMD.registerExtensionMenu("MyPluginTitle", startMyPlugin)
The example below is an extremely minmalistic Python script that creates a window when triggered by an appropriate selection in the VMD "Extensions" menu.
from Tkinter import * # A minimal plugin class that just creates an empty window class MyPlugin: def __init__(self): self.root = Tk() self.root.title("My Plugin Window") # Function to start the plugin. Must return the window handle. def startMyPlugin(): return MyPlugin().root if __name__=="__main__": import VMD # Register the plugin so that it's not actually created until the # first request to open the window. VMD.registerExtensionMenu("myplugin", startMyPlugin) # Create the plugin now and add it to the Extensions menu. # VMD.addExtensionMenu("myplugin", MyPlugin().root)