first of all, there is no plaintextemail.php

secondly, your screenshot is html format email, so even if plaintextemail.php existed your edits would do nothing there

what exactly do you want to display there anyway ?

perhaps using the “templates” options would be what you need / can use anyway instead of messing around with files (where you should be using action/filters anyway for this sort of thing instead of editing them directly)

many other wasy to do this, but this will probably do. tweak as required
in your themes functions.php add

add_filter('wppizza_filter_htmlemail_head','my_address');	
function my_address($markup){
$markup.='<tr><td colspan="2">
my address 1<br />
my address 2<br />
my address 3<br />
</td></tr>';
return 	$markup;
}