Help! I can't find my Media Player app!

I'm sorry, I think you misunderstood. The media isn't the problem. The files are avi and mkv and it's a USB 2.0 flash drive so it should be supported as of your link. What I'm referring to is the app to play the files itself. I can't find the app in the
Xbox store that plays the files I'm talking about. The app I saw on YouTube when I searched my issue through Google was called media player preview, but I can't find that app in the Xbox store. If I could get the app, playing the files would be a nonissue.
Thanks for your response though, any further help is greatly appreciated.

I believe the App should be on your console by default it's just called "Media Player". I was able to search it in the store on my Xbox itself as "Media player" so you should be able to find it there

Thanks for the help. Is there even a media center type app for XB1 yet? I can't find anything that is comparable to the Media function that you could get to on the 360 by hitting the guide button.

I'm having the same problem. I was working up until a few weeks ago and now nothing. Media Player sees my Windows 7 machine, but when you open it you get files not found. I've tried rebuilding the libraries and even reinstalling Media Player 12 but still
nothing. I'm currently using Plex media server on the PC but that seems to be 50/50 as to whether is shows up at all in the Media player app.

This is further complicated as my 360 also shows the PC but also can't find any files. I'm 99% certain the issue is Windows media sharing related but can't nail down what or why. I've been through the usual disabling firewalls, checking allowing media streaming
is enabled in MP12.

Ok, using DLNA I can add my NAS on XBOX media centre. However I can't open anything because the Media Player app doesn't ask for the login or password.