That's trying to use TemplateDOC
as the name of a file. Assuming your resource value is actually the XML itself, you want
base.document = XElement.Parse(RStudio.Properties.Resources.TemplateDOC);
I suspect that really is what you want - because if it's an embedded resource, there isn't really a filename (or URL) you could give it to load.
If it's were an embedded resource as a separate file in the assembly, then you could use Assembly.GetManifestResourceStream
and then XElement.Load(Stream)
.