I just replied your email.
The problem is much easier than you think.
It is a simple CHMOD issue.
This file below is stored the template configurations:
ROOT/templates/flexible_darkmart/config
(without a file extension at the end)
Depends on your changes in the "template confguration" through joomla backend, that file is updated. But since your server doesn't give permission to this file, the "write"access, your settings can't be saved through "template configurations". This is why you keep seeing the "warning" in the frontend even though you typed the username and order-ID. It is because your inputs in the "template configuration" has not been saved. This is the reason!
The solution is easy; open your FTP server and give this file a "write" access, try to change the CHMOD to 755 or 777, make sure it is not "644". And re-try to save the template configuration.
In the worst scenario, if you can't able to give the "write" access to this file, you can manually edit this file (through notepad). Open it, you would see something like this at the beginning:
"flexible_username":"","flexible_ordernumber":"","profile_default":"default".........
Type the Username and Order-ID in the blanks between quote-quote (""):
"flexible_username":"YOURUSERNAME","flexible_ordernumber":"YOURORDERNUMBER","profile_default":"default",.......
Then save and upload (overwrite) to your server. This would also work.
Regards.