There is still no possibility to generate GS1 datamatrix in Adobe Live Cycle Designer 11.
First option: use GS1 Code128 using the Command [F1] inside the string to be encoded.
Second option, if you are good in javascript, you can generate a gs1 datamatrix barcode as image inside the scripting area.
If you using SAP and Adobe Forms you could generate a gs1 datamatrix in ABAP as a image and transferring the image data as xstring over the forms interface to the form.
A Company who offers a ABAP solution for that are for example:
SAP Barcode Generator - Europe IT Consulting.ch