The only way you can enforce it is by bringing a legal action to both interpret (i.e. the proper time frame; getting a court interpretation of an agreement is called getting a "declaratory judgment") and enforce the agreement. Depending on how, procedurally, prior proceedings (e.g. the dissolution) were left, you either bring a motion in the prior legal matter or file  a new lawsuit to enforce the agreement. You are strongly advised to retain an attorney to help you navigate the procedural rules of doing this, which is more complex than, say, the average small claims case. Unfortunately, you will have to go to court; only courts can enforce agreements.