I personally think that ModSecurity is very important, but I'm biased as I am part of it development team. You may want to read more about the application security problem by browsing the OWASP top 10 http://www.owasp.org/index.php/Top_10_2007). ModSecurity is your best solution, and the only free one, for protection from many of the problems in this list.
We can also try to help you solve your specific problem with ModSecurity on the ModSecurity mailing list: https://lists.sourceforge.net/lists/listinfo/mod-security-users.
~ Ofer Shezaf
ModSecurity Core Rule Set project leader