There are so many different ways to go from Illustrator to the router. You do need something that inputs a path (usually a DFX or SVG) and allows you to define what the router does with it…engrave, contour cut, pocket, etc. and generate a g-code file. And you need a way to move that g-code to the control board of the MPCNC. Estlcam and Repetier-Host are only two programs of many possible solutions. There is a chart in this topic that maps out most of the pathways.
P.S. And there are other ways to use the router such as relief carving that require something other than Illustrator to produce 3D models as input.