The bulgarian-english pair can be found this location:

https://apertium.svn.sourceforge.net/svnroot/apertium/nursery/apertium-bg-en/

The apertium-bg-en.bg-en.dix file is simply a copy of the The apertium-mk-en.mk-en.dix, 

and it needs to be translated to bulgarian from macedonian.

For this task, pick a file from the cgi-tasks folder (each contains 500 lines, except for the last one which contains a bit more), and translate the source side from macedonian to bulgarian. Upload the translated file here.

Further instructions on IRC: irc.freenode.net #apertium