Who can file? In our divorce decree the judge signed it says I get half of my ex's 401k. Can I file the quadro or can only he file it?

It's a QDRO See http://www.dol.gov/ebsa/faqs/faq_qdro.html

You need to send a copy of the judge's order to the plan administrator or his employer's Benefits department. They will let you know if there is any additional paperwork you need to file.

Either of you can file for it.