Having populated the fields for the database sid, Oracle home, username and password and pressed Auto Discover, the auto discovery fails to find the database.
If the wrong build of the Oracle plugin is installed then Auto Discovery will fail.
Check the Oracle build for 32 or 64 bit and ensure that the plugin matches. I.e. for 32 bit Oracle use the 32 bit plugin and for 64 bit Oracle use the 64 bit plugin.