The adding a device manually is pretty straight forward, so I’m wondering if something else might be going on. What version of the app are you running?
Can you try the AIR version found here to see if it works? – http://simplehomenet.com/Downloads/iSmartenit.zip
When you download that zip, just unzip and double click the installer file to install it. You’ll need Adobe AIR on your computer if you don’t already have it – http://get.adobe.com/air
Just in case you might not be selecting something correctly, please review the manual device addition on the wiki page for iSmartenit Areas – http://wiki.smartenit.com/index.php?title=ISmartenit_Areas
If you’re still having trouble, please let us know.