MODIS data for atmospheric products is available from several NASA websites. The primary location is the ladsweb site http://ladsweb.nascom.nasa.gov/
Within ladsweb there are several ways to access the files depending on your needs. A sample exercise that will guide you through most of these is available on our educational and reference materials page.
More advanced users may wish to use the FTP site for accessing MODIS files, at ftp://ladsweb.nascom.nasa.gov, accessible via most internet browsers or FTP clients.