well this will probably be a long answer. you have to answer some questions first. when you added on the name did you do it in the cpanel. i will assume you did as there is a tutorial on add on domains. now you need a separate folder on your hard drive called
bmwexpertbuyer - in this folder you should have an image and a media folder.
and this same folder should exist at the server. you can add that folder using your ftp tool.
now that you have the folders done, you need to make an index page for your new site.
when you save this index page you save it to the proper folder on your harddrive. and when you publish this you should have it published to:
http://www.youroriginalname.com/bmwexpertbuyer.com
hope this helps, watch the video a few times.
each page after the index should be saved in the correct folder and published just like you did the index page.