Shortcodes are disabled by default in Mailster and will get removed from your campaign. The reason is that short codes can inject almost any html which may break the whole email.
If you like to enable shortcodes in Mailster add this function to your themes function.php
add_filter( 'mailster_strip_shortcodes', '__return_false' );
This will keep you short codes intact.
You may have to remove the line again if you get unexpected results in email clients.