In a civil case against the employee I can surely state that you should contact the old employer and advise them that you will not appear unless you are compensated for the time lost from work.  But in a criminal matter you may not really have that kind of leverage.  The DA can subpoena you and if you do not appear at the trial as a witness then they can have you arrested (they will issue a warrant and have you dragged in).  I would, though,speak with the DA and explain your plight and let them know that you need to be compensated.  See where that takes you.  And yes  would contact your old employer as well.  The DA can not compensate you.  Good luck.