At first please create a template under your reseller account (on ibuildapp.com), you may do it here: https://ibuildapp.com/mytemplates.php
After that you will be able to switch it on/off for your reseller site, with special checkbox:
Also you may manage your Templates on the reseller site, to so go to /wp-admin/apptemplates.php on your reseller site
Here you may manage templates. Your templates will be in the Custom Templates section.