If you are using Windows XP the previous advice will most probably not help.

Download the Bulgarian phonetic keyboard from
http://injinera.narod.ru/downloads/BG_KeyboardsXP_V.3.1.zip
Unzip in some temp directory. Right click on "BGPhV3XP.inf" and chose install. 3 new keyboards will be installed on your computer. Later you could choose the one "BulgarianPhoneticV3 WB by Genadi Atanasov-Injinera". In the subdirectory HELP (from the ZIP file) is shown how to do it and the difference between the 2 new phonetic keyboards.
Good luck!