http://www.ctan.org/tex-archive/biblio/bibtex/contrib/chicago*

Place it in a file called chicago.sty in the TeX search path. (Placing it in the same directory as the LaTeX document should also work.)

! LaTeX Error: Two \documentclass or \documentstyle commands.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...

l.101 \documentstyle{
                     chicago}