How to correctly use path in LoadFromFile method?

Well...yes. Why are you surprised?

When you specify a folder directly:

g.LoadFromFile("C:\\Users\\...\\kv.rdf");

you are saying where the file is.
When you reference it via MapPath from the File Upload control, all you get is teh file name: no path information (because you can't access the client hard drive at all and so the folder info isn't transferred fro security reasons).

So

MapPath("myFile.rdf")

will give you the equivilent of:

@".\myFile.rdf"

Where "." is the current folder as defined by the location of the current web page. If you haven;t explicitly saved the file there, the file will not be found...
Most likely, you want to use something like:

MapPath(@"~\Files\RDF\myFile.rdf")

Which will return a "Proper" path based on your website folder structure.

Thank you both. My solution is:

fuRDF.SaveAs(Server.MapPath("~/dat.rdf"));
string filePathDat= HttpContext.Current.Server.MapPath("~/dat.rdf");
g.LoadFromFile(filePathDat);

Thanks again! :)