What to do if my ex-husband is taking me to court for not following our parenting plan?

You don't specify if your parenting plan is per a current court order or just an agreement between the two of you-- regardless, it looks like he is either filing a custody suit or a modification of a custody suit.  Either way, if you have been served with notice of the suit, then you need to file an answer.  If you do not file an answer, a default judgment could be entered against you... which means that it will be difficult to undo any adverse decisions by the court.  

If you have not been served, you still need to pay close attention to the suit because of what you mention regarding him stating "he doesn't know where I live."  This sounds like he is setting up a basis for alternative service.  To avoid him getting a judge to sign a motion for alternative service, you can go ahead an file a general answer-- or start looking for a family law attorney to help you file a response.  The filing of an answer will keep him from getting any other motions granted without giving you proper notice.

In the answer, the various mistakes which affect legal issues can be highlighted.  For example, it sounds like his method of service on you may be defective.  You can also file a cross-motion within your answer for enforcement of child support.  If he's going to drag you back to court, make sure that you take advantage of the opportunity to enforce his obligation.  You can also ask the court to modify his visitation schedule in light of his recent, lack-luster, visitation history.

You are not required to have an attorney to file a response.  However, it usually is a good idea to have counsel when you are dealing with the stability of your children and an idiot who thinks they know the law.  A family law attorney will be able to shut down many of his antics fairly quickly.   If you cannot afford an attorney, you have a couple of options.  The first is to find an attorney that accepts payments or credit cards.  The second is to hire the attorney for the limited purpose of drafting or proof-reading your response.  A third option is to research non-profit legal organizations in your area to see if you qualify for free or reduced fee representation.  Your local clerk will often know of programs or clinics in your area. 

The main thing you should not do is panic or give up.  You have options-- just take the time to sort through which ones fit your situation.

I suggest that gien the concerns that you have as to the upcoming child custody hearing that you have written about that you immediately consult with a family law attorney to protect your interests and prepare for the upcoming hearing given the irregularities that you have written about in the pleadings.