How to manualy update post meta for WPAlchemy metabox

https://www.farinspace.com/wpalchemy-metabox/
I’m using this plugin and i need help!

So, I’m trying to do:
I have a user registration. When someone registers – creates new post (post type = plugin_users).
in functions i have defined fields for that custom post type (using the plugin wpalchemy).
So the problem is next:
in my save function i recive all data that need’s to be saved, even with the same meta_key that has wpalchemy created..
i’m creating new custom post with title of 1 field value.
and then, when post is created i wont to update_post_meta..
The way i tryed: store the post_meta key as i get from plugin and post meta value as i enter on registration..
the problem is that wpalchemy creates different post_meta:
it creates {_slug} as meta key and meta data as some kind of array (or i don’t know what.. ther’s planty of text..)
a:7:{s:5:"email";s:18:"[email protected]";s:4:"pass";s:3:"123";s:5:"pass2";s:3:"123";s:7:"surname";s:14:"Семёнов";s:4:"name";s:12:"Михаил";s:6:"region";s:29:"Санкт-Петербург";s:5:"group";a:1:{i:0;s:1:"1";}}

So the problem is that i cant manually update single post menta value not entering the post in admin and making changes there and pressing update…

Is there a solution how to save meta value that i need not from “edit post” ?? (from front end via ajax)