Working with custom fonts in Windows Store and Windows Phone projects requires you to know the 'full font name'. Make sure you get the name completely right, otherwise it wont work! Most of times the name in the character map tool built in to Windows works. - How ever sometimes it does not display the right one, depending on how you have installed the font.
To aviod having a TTF font not working or a OTF font not working in Windows Store or Windows Phone, I recommend using this tool http://us.fontviewer.de/download.html to the right full font name.
In case everything else fails: Remember you always enable 'copy always' for the files, and set the build properties to Content
In xaml you can create font families and reference them on your page:
<Page.Resources> <FontFamily x:Key="FontBook">ms-appx:////Styles/Fonts/FuturaStd-Book.otf#Futura Std</FontFamily> <FontFamily x:Key="FontHeavy">ms-appx:////Styles/Fonts/FuturaStd-Heavy.otf#Futura Std</FontFamily></Page.Resources>
If you are facing any problems working with fonts - let me know on twitter, and I'll help you out.