The purpose of the RewriteBase directive is to indicate what url-path needs to be added back when the rewrite is done. So the directives you posted assume the .htaccess file is in /home/username/domain/concrete/ directory where apparenty you installed the framework?
Second if you are uploading the file you may have a format problem if you are uploading it as binary instead of text. If you are familiar with logging in to a shell and using CLI you can run the dos2unix program to modify the file. Or when uploading have your FTP force text/ascii mode.
Also, here's a tip. When something "doesn't work" when you tell somebody else you need help you will want to mention what happens instead of what you expected. Most of the time if the web server software (DreamHost uses Apache for shared hosting) "doesn't work" it will show an "internal server error" page but it will put details in a log file. So when that happens you want to check the log file at /home/username/logs/domain/http/error.log and then report what it says.
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