You don't need an app. You just do it through the web browser on your phone.
1. Make sure you have the current build of MC15 (above).
2. Go to Tools -> Options -> Network and check the "Share" checkbox under the Library Server part of the dialog.
3. Take note of the TCP Port listed in the dialog. You can change this if you want. If you want to be able to access your library from outside your LAN, you will need to forward that port on your router. If your iPhone is on the same wireless network as the computer, then you shouldn't have to worry about this (though you may need to open the same port in the Windows Firewall).
4. If you don't know what it is, figure out what the internal IP address of your computer is on your LAN. Open a command prompt window and type "ipconfig" to find out.
5. In your web browser on your phone, open this web page: http://<MY_MC_COMPUTER_ADDRESS_HERE>:<TCP_PORT_HERE>
Assuming you don't have a firewall blocking the connection, this will open the web page being served by MC, which will give you the options to use WebRemote or WebPlay (or MCWS, which you don't need).
WebRemote: Allows you to control a running instance of MC15 with a web browser. Allows you to navigate your library, start and stop files playing, pause, control volume, navigate playlists, and all sorts of other wonderful things. It works QUITE well currently.
WebPlay: Basically the opposite. You can connect to your MC library and browse it on your phone and actually play the files on the device itself (rather than on the remote computer). This feature is still brand new and is much more "beta" quality. Works best with MP3 files.
Each of these navigation methods can have their own set of view schemes defined, just like you do for Theater View. These schemes are defined in that same Options -> Network dialog.