Captcha doesn't come out in my multiform webform
admin forms is not the same as webforms. You need to manually add the webform to the captcha settings. 

Go to admin/config/people/captcha/captcha/settings in the blank text box under "Form Protection" enter the form_id of the webform and save the configuration. Go to the webform page and you should be able to see the captcha there.

To quickly find the form_id of the webform, take the node id and append it to "webform_client_form_", for example if the node id is 3 your webform form_id will be "webform_client_form_3". If this is not your case, you can always use firebug to inspect the hidden form_id field.