Hopefully you are pressing Shft-P as "p" does indeed nothing.
I was using KeyRemapper to map the play/pause key on my remote to a key on my HID keyboard, and then trying to link that to MP using Input Manager. I eventually got play/pause working (the actual key I used to map to didn't really matter). However the results were very hit and miss and I was not able to successfully map all media keys using the same method.
I already knew that MP2 is designed to work with an IR MCE remote, however I don't need one of these for my main HTPC as it is simpler for me to use the mouse and keyboard, but for my bedroom setup I thought a device which was a media remote, HID keyboard and air-mouse all in one was an elegant solution, and would save having three separate devices laying around. I just assumed (wrongly) that I would be able to map all the keys on any HID device to functions within MP2 using the input manager, but it appears that while Input Manager supports the "normal" keys which you can find on a standard HID keyboard, the media and shortcut keys are not recognized. Whether this is a limitation of Windows, or an omission in MP2 I do not know.
If its not a bug in Input Manager that these keys aren't recognized, or if it is not possible to add support for them, then that's fine. I will just find a different solution, like maybe a normal wireless keyboard with a touchscreen built in.