some important corrections, forgot, to explain it in my first post....
mpform ask PHP for the gd-extension (see PHP-Info in the WB-Info-Windows and there a special section called "gd", if installed")
the add.php ask for this section and send's true or false as answer. TRUE is like 1, so no problem, but FALSE is nothing, because, the script needs a integer value, but get nothing as answer and because of this, you get a error message, if you add a mpform-section
if you set the database value from 0 to 1, like my answer in the top) and the gd-library is not installed on this server, you see no captcha picture in the mpform, because, the picture will build "on demand" with the gd-library and put there calculation for the captcha to the background-images.
So without a installed gd-extension on this server, you cannot use the captcha in your mpform. you can set it in "advanced settings" from mpform to DISABLED in the setting "use Captcha"