To be able to use __uuidof() a uuid must have been declared for the target. When compiling for Windows 8 Store Apps it seems that MMDeviceEnumerator and IMMDeviceEnumerator don't have any uuid declaration and you should instead use the Windows::Devices::Enumeration namespace to enumerate the devices and then use IActivateAudioInterfaceAsyncOperation to initialize and activate the audio device.
Here is a link to a fully working example that captures pcm data and writes it to a wav-file. It uses the systems' default audio capture device but enumerating the available devices and using any of them instead is a piece of cake. :)