How to go to particular Item in IEnumerable
var item = eLevelData.ElementAt(index);
If your collection is typed as IEnumerable instead of IEnumerable<T> you'll need to use the Cast extension method before you can call ElementAt e.g.
var item = eLevelData.Cast<RMSRequestProcessor.RMSMedia>().ElementAt(index)
Don't know much about what subset of .NET BCL/LINQ is available in Silverlight, but Skip should do the trick. But generally speaking it still uses foreach internally:
var item = eLevelData.Skip(4 /* or 5 */).First();