How to hook to content profile edit form validation
It will not work as it is not a hook.

Try hook_form_alter() to edit form so that $form['#validate'] can be defined.

$form['#validate'] = !empty($form['#validate']) ? $form['#validate'] + array('mycustom_validate_handler') : array('mycustom_validate_handler');


Then define your function

function mycustom_validate_handler($form, $form_state){
     // Do your validation.
}