That message appears when you post data to a vb script without including the security token value in your form. If the page with the form is a vb page made from a template you could use this:
Code:
<input type="hidden" name="securitytoken" value="{vb:raw bbuserinfo.securitytoken}" />
Or if you're including global.php in your page you can get the value from $vbulletin->userinfo['securitytoken'].
If you need any more help you'll have to explain what you're doing in more detail.