Changing validation messages

I have a simple signup form using GF registration plugin as a widget.
Form works fine when submitted with correct content but I need to change the generic error messages.

I have changed the validation fields on the form itself, but these are not displaying - just the default message.

I have added

<?php
add_filter("gform_validation_message", "change_message", 10, 2);
?>

into the theme functions.php but I just get this error:

"Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'change_message' was given in /home/MYSITE/public_html/wp-includes/plugin.php on line 170"

All I want to do is change the validation messages in a simple form!

This is all using latest WP and GF.

Cheers.

Not sure why it didn't work before but that changed the validation title this time but the field error messages are still as default.