While you can freely use macros in the Equation Editor, there is no facility for entering or changing them. To enter and alter macros, the Text Editor must be used. To do so, close the Equation Editor and select View>As Text from the Sketch Editor's menu. Add and alter Macros as needed, then select View>As Sketch from the Text Editor's menu. All of the sketch information will be retained through this process.
As a general rule it is probably wise to define useful macros at the time you create the model. This will keep the model development process much simpler. Generic catalogs of macros can, if desired, be included in every model, to be invoked when needed.