The control supports loading from arbitrary stream (file, in memory etc.) so you simply need to convert the string to a stream - for example:

string someRichText = @"{\rtf1\ansi\ansicpg1252\deff0\nouicompat\deflang1033{\fonttbl{\f0\fnil\fcharset0 Calibri;}}{\colortbl;\red255\green0\blue0; }{\*\generator Riched20 10.0.14393}\viewkind4\uc1\pard\sa200\sl276\slmult1\f0\fs22\lang9 Rich \cf1 Text\cf0\par}";
byte[] bytes = ASCIIEncoding.Default.GetBytes(someRichText);
MemoryStream memoryStream = new MemoryStream(bytes);
nRichTextViewControl1.Widget.LoadFromStream(memoryStream, new Nevron.Nov.Text.Formats.NRtfTextFormat());
Hope this helps - let us know if you meet any problems or have any questions.