EndNote Online is compatible with both Windows and Macintosh machines. If using a Mac, you may find it easier to use Firefox as your browser rather than Safari.
If you use Mozilla Firefox as your browser exporting from databases on a Mac. (you can download it from http://www.getfirefox.com) you will find it easier to either save the file for import or if direct export is available you will find it easier to select the EndNote application.
According to the EndNote help, there are known problems with the way Internet Explorer, Chrome and current Safari browsers on a Mac OSX system handle MIME types, and this causes problems with the ability to directly export from certain websites into EndNote.