You can't enable anonymous FTP under the DreamHost system unless you have purchased a unique IP address for your domain, at a cost of $3.95 per month.
One option would be setting up an FTP user specifically for this purpose. Uploads would go into this users home directory, keeping it relatively isolated from your web site content etc. Of-course, you would also need to log-in using this users credentials to access the uploaded files.
You could do as BUGabundo suggests and install some kind of upload script, a search of hotscripts.com should turn up a few suitable for the purpose, but remember that if the script is written in PHP it will be subject to an upload limit of ~7MB, due to the default settings of PHP. This limit can be overcome, but the process to do so is a little involved and should only be attempted if you are comfortable with playing around in the shell.
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