[Typo3] MailformPlus and Captcha

[Birk]

Hi List,
I trying to use MailformPlus with the Captcha EXT but can't figure out 
how to get it to work.
I have a page with the mailform and and extention template.
The mailform sends mail ok but the captcha doesn't show.
This is what I have:
#setup
:plugin.tx_thmailformplus_pi1.captchaFieldname = captcha_check
In the template:
<td><input type="text" name="captcha_check" value=""> ###CAPTCHA### </td>

I must be missing somthing simple here!
Thanks,
L.