I installed TWiki here on DreamHost with very little fuzz, by following the excellent instructions that Richard Donkin wrote: http://twiki.org/cgi-bin/view/Codev/DreamhostSetupNotes
Specifically, there's a part that reads:
To avoid permissions problems:
* Type ls -l from within your CGI bin directory, and check that the owner (you) has rwx or r-x on every script, and that group and other have r-x or r--.
* If any permissions are wrong, type chmod u+rwx,g-w * within this directory.
* (Optional) Type chmod go-rx * within this directory to ensure nobody else can read or execute your scripts.
Take a thorough look at those instructions, and if you still have questions, you're more than welcome to contact me or Richard.