Now you've got the compressed tar file that contains the BIND source. Just use zcat to uncompress it, then untar it:
zcat bind-8.1.2-src.tar.gz | tar -xvf -
(This assumes you've got a version of zcat that can handle gzipped files; if you don't, go get one via anonymous ftp from alpha.gnu.ai.mit.edu in /gnu/fileutils-3.16p.tar.gz.) This will create a src directory with several subdirectories, including bin, include, lib, and port. They contain:
Source code for all BIND binaries, including named.
Copies of include files referenced by the BIND code. You should use these to build your name server instead of the ones shipped with your system, since they have been updated.
Source code for libraries used by BIND.
Information BIND uses to customize compilation settings and compile-time options for various operating systems.