Step 1 
Add a flow into the foxes compartment, and rename it fox repro.

Step 2 
Add a flow out of the foxes compartment, and rename it fox mortality.

Step 3 
Draw an influence from the rabbit’s predation flow to the fox repro flow.

Step 4 
Enter the following expression for the fox repro flow: 0.01*predation

Step 5 
Enter the following expression for the fox mortality flow: 0.2*foxes

Step 6 
Rebuild the model

Step 7 
Set up graph display for the foxes compartment
Note that you need to have a separate plotter display, since the number of foxes is typically a couple of orders of magnitude less than the number of rabbits. If you plotted them on the same graph, the graph line for the number of foxes would be right on the X axis. 
Step 8 
Run the model again
You should now see that the rabbits and the foxes compartments cycle, with the rabbits population crashing as the foxes population increases, followed by a crash in the foxes population. 
This reflects the assumption in the standard textbook model, that the predator reproduction is proportional to rate of predation on the prey.
Your model diagram should now look like this: