XceedStreamingCompressionLib.XceedStreamingCompression compressor =
                 new XceedStreamingCompressionLib.XceedStreamingCompressionClass(); 

compressor.License( @"your license key" ); 

compressor.CompressionFormat = new XceedStreamingCompressionLib.XceedBZip2CompressionFormatClass();

object bytesRead = null;

try
{
  object compressedData = compressor.ReadFile( @"c:\test\file.txt", 0, 0,
                      XceedStreamingCompressionLib.EXCFileProcessing.cfpCompress, true,
                      ref bytesRead );               
}
catch( System.Runtime.InteropServices.COMException except )
{
  MessageBox.Show( except.ToString() );
}