Unfortunately, I do not see a way to get what you want from within the Fit Model results. Graph Builder, as indicated by @txnelson , has the ability to create the graph that you want. Unfortunately, that is not tied to a model, so you get a contour plot that is created by triangulation. This can lead to the rough appearance when you have a sparse amount of data.
So, what to do? You need a large dataset with the model so that Graph Builder will give you the smooth contours. Here is one way to do that:
From your Fit Model results, click on the Prediction Profiler red popup menu and choose Simulator (the Simulator is also available from the Contour Profiler menu).
In the dialog below the prediction profiler, change the setting from Fixed to Random > Uniform for the two factors that you want on the contour plot (check to make sure the ranges are appropriate for those factors). The other factors leave at Fixed, but put them at the setting that you wish for them to have.
Choose the number of runs for the simulation. Leaving at the default of 10,000 is probably sufficient. Under the Simulate to Table outline, click on Make Table.
You now will have a new table that you can use to create a contour plot in the fashion that you would like.
Dan Obermiller