The address to the formmail script should be cgi-bin/formmail.pl and not the full URL of the site
The form should have the following fields:
<form name="form1" method="post" action="/cgi-bin/FormMail.pl">
<input type="hidden" name="subject" value="Title of the email subject" />
<input type="hidden" name="redirect" value="Full URL including http of the 'thank you for contacting us' page" />
<input type="hidden" name="recipient" value="email address of the recipient" />
You can specifiy most of this in the script if you know how, but the above is probably the simplest method for getting it to work.
The Formmail script itself should have this for its first line [sometimes differs from one host to another]:
The next unxommented line is the address of your email program script on the server:
$mailprog = '/usr/sbin/sendmail -oi -t';
@referers = ('your site URL without www','your site URL with www','you can use localhost as well if required');
@recipients = &fill_recipients('your site URL without the www','again you can leave localhost in if required');
And that should get you running.
You can alter and configure lots more but normally you don't need to and the above will take pretty much any form field type you could mention and process it correctly when passed through the Formmail.pl script.
One last thing: you may want to rename FormMail.pl to something else like contact.pl or whatever, then change the reference in the form to point to that file in your cgi-bin.
FormMail.pl is often targetted by spammers and renaming the file will help to prevent this from happening on your web server.
Hope that helps.