Here is what you would need to do. Create your load test, adding the tests (unit, web or otherwise) that you will be executing, and the machines where the appfabric cache is located among any other SUT resources you will need to monitor during the test.
The AppFabric counters are not part of any of the default counter categories, so you will need to add them manually. You can do this by editing the load test, right click on Counter Sets, and select Add Custom Counter Set, and name that set AppFabric. Next,
right click on that and choose Add Counters.
From the dialog that opens, select your AppFabric server as the computer, and you can select the AppFabric counter categories to complete creating your counter set.
Finally, you will need to go to Run Settings, and enable the AppFabric counter category for each of the AppFabric servers.