Are you including the font file in your app package?
1) Add the font file that you wish to use to your project, and set the Build Action to Embedded Resource. Also ensure you are copying the file to the assembly in the "Copy to output directory" property. 2) Select your textblock, and pull down the font selection dropdown in properties.
You should see your font there, and it will be compiled into the assembly and available for use in the app.