You cannot manually add computers to the database. The Devicetester.exe is only used for testing purposes as well and does not pull any data into your database. The "Rescan" and "Full Rescan" buttons only work for computers that are already in your database.
If you have this PC with you and own a Lansweeper license, we would recommend simply running LsPush on the machine in order to scan it. Copy the LsPush.exe located in Program Files\Lansweeper\Client on your Lansweeper server to the client machine, run it locally without any extra parameters and place the resulting scan file in the Program Files\Lansweeper\Service\import folder on your Lansweeper server to import it.