Taken from the LyX help file and adapted for the given circumstances:
Start by copying the existing class's layout file into your local directory (I assume the underlying class is report and we work in Linux):
cp report.layout ~/.lyx/layouts/acl2012.layout
Then edit acl2012.layout and change the line:
\DeclareLaTeXClass{report}
to read
\DeclareLaTeXClass[report, acl2012.sty]{report (acl2012)}
Then add:
Preamble
\usepackage{acl2012}
EndPreamble
near the top of the file.
Start LyX and select Tools->Reconfigure.
Then restart LyX and try creating a new document.
You should see report (acl2012) as a document class option in the Document->Settings dialog.
In case of complications (sectioning commands working differently etc.) consult the LyX help file that contains further information.