Hey Sheila,
I figured this out today.. so I thought I'd see if you were still needing help.
In order to do this, you use jQuery and manipulate the form using the directions here: http://developers.marketo.com/documentation/websites/forms-2-0/
So you would alter your embed code to look like this:
<script src="//app-sjqe.marketo.com/js/forms2/js/forms2.js"></script>
<formid="mktoForm_621"></form>
<script>
MktoForms2.loadForm("//app-sjqe.marketo.com", "718-GIV-198", 621, function(form){
jQuery(document).ready(function($) {
$('.mktoButton').text("new text here");
});
//from here we have access to the form object and can call its methods.
});
</script>
And voila! You've changed your submit button text.
Please note: You need to change the variables in there to match your form you're embedding. Also, I'm using this setup on a Drupal-based website, so my document ready function has to work around some peculiarities with Drupal -- depending on what you're using, you might not need to use that, or it might work all the same.