I'm not sure what configuration you will need to do as I have never used sendmail on Mac OS X. To start sendmail you will need to modify permissions on the root folder as sendmail does not like the default permissions.
To do this either su to root or use the following sudo command (from terminal.app):
sudo g-w /
(use your admin account password when prompted).
This removes group writable permissions from the root directory.
Now start sendmail:
sudo /usr/sbin/sendmail -bd -q15m
To have sendmail start automatically on reboot edit the file /etc/hostconfig and change the -NO- to a -YES- on the MAILSERVER line (must be root to edit this file).
Also, if you want sendmail to queue every 15 minutes instead of 1 hour (the default), edit /System/Library/StartupItems/Sendmail/sendmail and change the -q1h to -q15m (must be root to edit this file).
Then make sure the path to sendmail is correct in your php.ini file (/usr/local/lib/php.ini usually). Make sure that sendmail_path=/usr/sbin/sendmail and remove the ; from the beginning of the line.
Restart Apache and give it a shot!
Jon