The act method in MainMenu needs:
in it with implementation for the help button to work.
  if (Greenfoot.mouseClicked(btnHelp))
{
    ...
}if (Greenfoot.mouseClicked(btnHelp))
{
    ...
}if (Greenfoot.mouseClicked(btnHelp))
{
    ...
}    public void act()
    {
        if(Greenfoot.mouseClicked(btnStartButton))
        {
           //start simulation when start button is clicked
           startSimulation();
        }
        if (Greenfoot.mouseClicked(btnHelp))
        {
            //show help menu when help button is clicked
            help();    
        }
    }
    
    private void startSimulation()
    {
        Greenfoot.setWorld(new BeeWorld(weatherInput.weatherValue)); 
    }
    
    private void help()
    {
        //add help menu
        HelpMenu HelpMenu = new HelpMenu();
        addObject (HelpMenu, getWidth(), getHeight());
        HelpMenu.setLocation(650,200);
        
        //button to be able to remove help menu
        RemoveMenu RemoveMenu = new RemoveMenu();
        addObject (RemoveMenu, getWidth(), getHeight());
        RemoveMenu.setLocation(900,200);
    }
}