Scripting the values in the selected dropdown menu

Both sets of code do seem to work, but with some caveats. It may depend on if you were selecting the Dropdown Menu Control effect or the actual Menu property, because selecting just the effect won't work with your code, as you would need to identify or loop through the properties in the effect to find the actual Menu property (it's actually property(1) of the menu effect). Also, on Mac at least you can paste into a prompt by right-clicking and selecting Paste there.

I'm also getting a scripting error if trying to do it onto a selected effect so that could even be causing your code to fail abruptly, and without warning if you're not using any kind of debugger:

"After Effects error: internal verification failure, sorry! {stream at index matchName="Pseudo/@@VZ4L33hTQ+C834jJeOfA5w" (mismatch with resultP, expected "Pseudo/@@i5MOg14MRDOoK5K/CfQpaw")}"

In the following code I first check if it's a dropdowneffect, and if not I check if it contains multiple properties (i.e. it is the main effect property) and if so check if property(1) of that property is a dropdowneffect. I also wrap setting the dropdown in a try/catch to avoid the error this returns from killing the script. I think in practice, even if you did actually have the 'Menu' property selected, the 'Dropdown Menu Control' effect will always come before it in any selection so it will be the 'else if' bit that always runs first.

And a side effect of setting the dropdown is that it seems to invalidate everything in selectedProperties, so if you wanted to loop through multiple selections at the same time you'd need to make a note of that selection in some other way at the start so you didn't later need to rely on selectedProperties.

var dropdownItems = 

[
"Alex4D",
"East of England",
"Greater London",
"North East of England",
"North West of England",
"South East of England",
"South West of England",
"West Midlands",
"Yorkshire and the Humber",
"(-",
"England",
"Northern Ireland",
"Scotland",
"Wales",
"UK"
];

var i, j, dropProp;
var props = app.project.activeItem.selectedProperties;
for (i=0; i<props.length; i++) {
	dropProp = null;

	if (isValid(props[i])) {

		alert("props["+ i + "] isValid");

		if (props[i].isDropdownEffect) {
			alert("actual menu prop was selected. matchname = " + props[i].matchName);
			dropProp = props[i];
		} else if (props[i].numProperties > 0 && props[i].property(1).isDropdownEffect) {
			alert("drop down menu effect was selected. matchname = " + props[i].property(1).matchName);
			dropProp = props[i].property(1);
		}

		if (dropProp != null) {

			alert("dropProp is valid = " + isValid(dropProp) + ". setting menu items");

			try {
				dropProp.setPropertyParameters(dropdownItems);
			} catch(e) {
				alert("error= " + e.toString());
			}

			alert("dropProp is valid = " + isValid(dropProp));
		}
	} else alert("props[" + i + "] no longer valid");

}

Just saw Mylenium's reply. It might be he's thinking about actual scripting UI dropdowns as he mentioned visibility which is more of a scriptUI thing. I've never tried scripting on these new Dropdown Menu Control effects before but they definitely appear to be changeable, although there is this annoying scripting error that I have no idea why it's happening. Just another of the many scripting bugs in AE I guess.