get value of layer controller in aftereffects

This is broken out into a bunch of steps so it's easier to see, and note that there aren't checks to ensure that the index that you're setting or getting is valid-- just here to show a few possibilities!

(function () {
  var comp = app.project.activeItem;
  var layer = comp.selectedLayers[0];
  var effects = layer.property("ADBE Effect Parade");
  var layerControl = effects.property("ADBE Layer Control");
  var layerDropdown = layerControl.property("ADBE Layer Control-0001");
  // This value is the index in your current comp of the dropdown's target
  var dropdownLayerIndex = layerDropdown.value;
  // This is how you'd get the layer specified by the dropdown
  var dropdownSelectedLayer = comp.layer(dropdownLayerIndex);
  // To set the value of the dropdown, you can do this--
  // where the value passed in is a number from 0 ("none"), to comp.numLayers
  layerDropdown.setValue(0);
})();