Hi 3dcnc  midpoint snap is only intended to work on lines and arcs.
It tends to be a time consuming calculation to find the true mindpoint of just any wiggly curve, so in order to have good interactive snapping performance MoI does not try to calculate those.
But it is possible to generate a point marker at the midpoint of a curve though, which you can then snap to.
To do that, draw a point object (Draw curve > More > Point) at the start of the curve, then select that point, run Transform > Array > Curve and pick the curve as the path curve for the array. Then for the array options set Item count = 3 and push Done. There will now be a point object at the halfway distance along that curve which you can snap on to.
 Michael
