Thanks for pointing out this oversight. The embedded option does not use a template, which makes it a bit more difficult to hack. I will fix it for the next release (by adding a new constant which includes the link to accept/reject), but if you want to fudge it in the meantime, you would need to hack the /administrator/components/com_nbill/framework/classes/nbill.email.class.php file around line 426 where it says:
$footer .= "<br /><br />" . NBILL_EM_REGARDS . "<br />" . nbf_cms::$interop->site_name;
...replace with:
$url = nbf_cms::$interop->live_site . "/" . nbf_cms::$interop->site_page_prefix . "&action=quotes&task=view&id=" . $document->id;
$footer .= "<br /><br />To view your quote online and accept, partially accept, or reject it, please visit: <a href=\"$url\">$url</a>";
$footer .= "<br /><br />" . NBILL_EM_REGARDS . "<br />" . nbf_cms::$interop->site_name;
This is just a temporary measure (I would not normally recommend hard-coding text like that!) - it is probably safer to hard code the text rather than insert a new constant in the language file yourself because if I need to add any further constants to the language file, you could miss out on them during the upgrade otherwise (as the upgrader may think you already have the latest additions).