http://cg.tutsplus.com/tutorials/luxology-modo/quick-tip-adding-a-render-output-dropdown-to-your-ui-in-modo/

In this Quick Tip tutorial, we will continue on from where we previously left off with our lessons on customization, and take a look at adding yet another UI element to our custom form layout – the popup menu. We will use this, combined with Modo’s macro system, to create an easy way of adding Render Outputs to our shader tree.