The Prefix tells Wowza which Base Path to use so it knows where to look for the content as you can have multiple stores available.
With a Base Path of "http://<CDN HASH>.rackcdn.com/content/" assuming that you have sample.mp4 in this directory and you're allowing playback with Flash HDS, you should be able to play using the URL below.
I've just configured MediaCache with the following information:
Name Base: disk2
I can play this stream with VLC player using:
Under "Validate Configuration" I can see the following:
Enter a sample content file path/name you want to stream:
Using the Prefix, Base Path, and sample content you entered, the full address to your content is:
I'm not sure why the Validate Configuration says I have G:/data/media/sample.mp4 when I don't have a directory called data or media in this location but it can be ignored.
You will need to restart Wowza each time you edit the MediaCache configuration for the changes to take effect. This is not something which con be done using an application restart.
Hope this helps.