If you have an extension file which you might obtain from the internet, now how do you import it into your extension builder?
Log into your extension designer account
Select "My extensions" in the "My plugin" column on the sidebar and click "Import data" button at the top
Choose your .mext file to upload it
The upload process is as follows:
When the last step shows "successfully importing data" , it means the upload is done. Click the close button.
After closing the modal, the extension will be listed in your extension list (if the list is not updated, you can try to refresh the current page) , and you can click on the extension for further configuration. Isn't that simple!