As far as I know, there is no direct method for RadMenu to access the item using ID. RadMenu allows accessing items according to url, text, value there is GetAllItems method to access all the RadMenuItems, which is described in the documentation.
But, you can explicitly check for the item by iterating through all the items as shown in the sample code.
CS:
string value = "";
foreach (RadMenuItem item in RadMenu1.GetAllItems())
{
if (item.Attributes["id"] == "id2") // I set id attribute to each menuitem