Hi,
You should first get the rich text view widget of the NRichTextViewWithRibbonControl by using the GetFirstDescendant method of its widget and then you can use the LoadFromFile method of the rich text view to load a file. Here's a C# code sample:
NRichTextViewWithRibbon richTextWithRibbon = nRichTextViewWithRibbonControl1.Widget;
NRichTextView richTextView = (NRichTextView)richTextWithRibbon.GetFirstDescendant(NRichTextView.NRichTextViewSchema);
richTextView.LoadFromFile(@"C:\Documents\MyFile.html");