If you're on a Mac, you cannot embed fonts in Flex or AIR apps directly from the dfont files that come with the system. However, you can do this:

1. Install fondu (sudo port install fondu or get the OS X package)
2. Copy the fonts you want to convert from /System/Fonts to, say, ~/mydfonts
3. Switch the output folder (e.g., cd ~/mynewfonts)
4. In Terminal, invoke fondu (fondu ~/mydfonts/*.dfont)
5. In your Flex Builder project properties, add the following compiler tag: -managers flash.fonts.AFEFontManager (or else you'll get an exception during transcoding: Unexpected exception encountered while reading font file error)
6. Finally, embed the font as you would normally via CSS. e.g.,

   @font-face {
     src: url("fonts/Geneva.ttf") ;
     font-family: "GenevaAIR";
     advanced-anti-aliasing: true;
   }

The How to: embed dfont fonts on OS X in Adobe Flex and AIR apps article by Aral Balkan, unless otherwise expressly stated, is licensed under a Creative Commons Attribution-Noncommercial 2.0 UK: England License.