@gryphonsegg as David said in the post above
MyComboBox.set("items", ["Piano", "Flute"]);
will not work.
The items in a combobox is a string with a line break after each item like this were \n is the line break
MyComboBox.set("items", "Piano\nFlute");
that is a bit cumbersome to write if you have a lot of items so, often you write them in an array but then you have to use .join to assemble them together to a string like this
MyComboBox.set("items", ["Piano", "Flute"].join("\n"));