go to the bindings panel and expand the session collection. make sure therte is a session in the list named:
if if is not there, click the plus button and add it.
then on the server behaviors panel click the plus button and select webassist -> validation -> server Validation.
for the validation type, select like entry.
click the lightning bolt next to server variable and select the captcha_Log_in_group_Security_code session variable.
click the plus button to add it, in the next window you need to select the comparison element, click the lightning bolt and select the Log_in_group_Security_code form element.