Just to expand on what Jewels wrote;
Unlike many hosts, DreamHost does not configure sub-Domains as sub-directories of the parent domain. Each sub-domain on DreamHost gets it’s own web accessable directory in your home directory and for all practical purposes can be considered the same as a full domain.
For example; If you have the domain example.com already hosted on dreamhost and you add the sub-domain sub.example.com, your home directory will contain the following directories.
You can then upload the relevant site files to these directories.
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