If you are creating a website by hand, you just upload the file and then create a link to it.
You shouldn’t need a plugin, even in a CMS like WordPress, since they don’t read .mobi files. People would click on the link and the file would download to their device.
In WordPress, you can just upload the .mobi file to the media gallery. I’m pretty sure WP media can handle that file type, but I haven’t tested it.
BTW, your “Dreamhost account” is not the same thing as your website. You can have several websites with the same DH account.
To upload to your website you need ftp access and an ftp client like Filezilla.