You have to convert the Excel list to a single line that looks like this:
var l = this.getField("ListBox");
l.setItems(["One", "Two", "Three"]);
… where, instead of “ListBox”, enter the name of your field, and instead of “One”, “Two”, “Three” replace it with your list from Excel.
As described in the post I linked to, this should be added as a Document script in FormMaker, and that Document script should be removed (in Acrobat) before you distribue the final PDF.
This is a workaround, but it makes it a lot easier than using InDesign’s UI for the task.