You now need to add an alias to apache's httpd.conf pointing to
%%PREFIX%%/squirrelmail/ in order to access SquirrelMail from your
web browser, or create a VirtualHost with DocumentRoot set to that
directory.

For SquirrelMail to work properly you will need to make sure the
following options are set in your php.ini file:
register_globals = On
file_uploads = On

In order to do your administrative configuration you need to 
cd %%PREFIX%%/squirrelmail && ./configure
SquirrelMail will not work until this has been done.