problem with adscaptcha

I installed this plugin “AdsCaptcha” from http://www.minteye.com/
if you go to my website, http://faxplus.ca/about-us/
and go as your going to post a comment and the reg stuff comes up and fill it out, us the captcha and you will get this error Fatal error: Call to undefined function mb_detect_encoding() in /home/faxplus/public_html/wp-content/plugins/adscaptcha/adscaptcha.php on line 72″

I am not sure if its a wordpress reg problem or if its a captcha problem. please help.

thanks

http://wordpress.org/plugins/adscaptcha/