the plugin is fully WPML’ed.
that said, I also had this happen to me a couple of times that WPML just refused to register the strings for reasons unknown (to this moment anyway)

the way i found around this was as follows – (tested this a few times and seems to work just fine)

a) backup your site (always a good idea in case it goes wrong somehow)
b) in wppizza/classes/wppizza.actions.inc.php on approx line 754 you find a line that reads

//$forceUpdate=1;

.
change this to

$forceUpdate=1;

then reload some WP admin page.
now change it back to what it was. i.e

//$forceUpdate=1;

check your string translations. they should all be there now (again, at least that worked here the couple of times i had that issue).
furthermore, future updates to the plugin should automatically register any possible new strings provided WPML is already installed, so the above workaround should only have to be done this once only

let me know