How do I get a key from a OrderedDictionary in C# by index?

There is not a direct built-in way to do this. This is because for an OrderedDictionary the index is the key; if you want the actual key then you need to track it yourself. Probably the most straightforward way is to copy the keys to an indexable collection:

// dict is OrderedDictionary
object[] keys = new object[dict.Keys.Count];
dict.Keys.CopyTo(keys, 0);
for(int i = 0; i < dict.Keys.Count; i++) {
    Console.WriteLine(
        "Index = {0}, Key = {1}, Value = {2}",
        i,
        keys[i],
        dict[i]
    );
}

You could encapsulate this behavior into a new class that wraps access to the OrderedDictionary.