As WaltR stated, you need to install the codec pack. The link is obtainable from within the Hoster Manual, if you select shortcut where it reads (Important Make These performance Changes for ) then select Windows 10/8.1 hyperlink, the item no 2 (#2 instal video codecs) there is another Hyperlink to take you direct to the site.
Heres is that link:
http://www.free-codecs.com/k_lite_me...k_download.htm.
Choose the 13.2 the latest version click on the link directly not any of the other big download links that are trying to get you to install other software.
You will probably be asked if you want to install various other things once you try the install, my advise is to decline all the offers except the actual K-lite megapack and
Media player Classic as I think that will solve your 2nd problem for showing youtube videos.
Which program are you using as your default video player?
If you are using Windows Media Player change your default player to the one in the download
Media Player Classic as this has settings to allow you to go full screen to a second monitor without changing your display settings.
I cant Remember the settings at the moment as Im on holiday and dont have access to a second monitor but the settings from the View tab options/Full Screen where if you have your second monitor connected you can select it to play on that by default once you press the F11 key.
Ill be able to help more when Im home the weekend.