Login as an admin, then
Admin area > Forum settings >
CAPTCHA > [x] Contact form
CAPTCHA type [x] Mathematical
If you want to use the graph CAPTCHA, select [x] Graphical instead
Note that the graphical CAPTCHA requires the GD Lib and at least one TTF font. Put the font files (*.ttf) into the folder captcha/fonts/. You can get fonts e.g. on free-fonts.com.
If you are not sure whether 'GD' is running, fire up a text editor and prepare a small php-script:
<?php echo 'PHP version: ' . phpversion(); echo 'PHP info: ' . phpinfo(); ?>
Save the file as e.g. 'xyz.php', upload it to your webspace and go for the address in your browser. Search the page for 'GD'; if you don't find a line like 'GD Support enabled', you are in bad luck. In such a case talk to your ISP.
The script 'xyz.php' must be deleted form the webspace afterwards - it's a safety risk.
Alfie (Helmut Schütz)