How do I get the ID of the selected item type from the TOC?

I'm not sure exactly what you're looking for. If you wanted to get the id of a specific itemType in a JS method, you could do something similar to the following:

var newItem = inn.newItem("part","get");

newItem.setProperty("name", {YOUR ITEMTYPE NAME});

newItem = newItem.apply;

var id = newItem.getID();

This can get whatever ItemType you're looking for and extract the ID. Let me know if I've misunderstood your question.