I have had some questions about this topic and thought it a good time to post up a sample showing just how easy it is. Please download the sample here: DataMatrix sample.
The WPF team made this sample and it shows how to create the DataMatrix barcode.
String text = "TYPE 3523 - ETWS/N FE- SDFHW 06/08";
PdfDataMatrixBarcode dataMatrixBarcode = New PdfDataMatrixBarcode(text);
// rectangular matrix
dataMatrixBarcode.Size = PdfDataMatrixSize.Size16x48;
dataMatrixBarcode.XDimension = 4;
dataMatrixBarcode.Draw(page, New PointF(25, 300));