It took ages for me to configure native captcha in contact form in Magento Enterprise 220.127.116.11.
My task was to add native captcha to contact form, that is on /contacts/index/index url.
Native CAPTCHA settings
First of all find Customer Configuration and open CAPTCHA settings:
How to add new form to CAPTCHA settings?
Create new module and add this lines to config.xml:
You might found here module/observer. It is explained further.
How to show CAPTCHA in contacts form?
Find your form template and insert:
After that find layout xml for your page, for example mine was contacts.xml:
How to check CAPTCHA?
Create Observer for your module:
Keep notice that was said about config.xml:
Rename module/observer to your module name.
Rename controller_action_predispatch_routename_controller_method to your controller method. For example in my case it was controller_action_predispatch_contacts_index_index