In Mac and Linux do you enter that url?
On FAQ page from tortoise http is used (I only have used svn+ssh)
From the wiki http://wiki.dreamhost.com/Svn#Using_HTTP
- Use the HTTP path that you used in the Subversion panel.
- Sign in using username and password specified in the panel.
Is there any way for svn users to change their password in this mode? No. Only through panel.
Is there any way to use https instead of http? Yes, but first you will need a unique IP address and Secure Server installed on your domain.
So I think maybe you have problems with authentification....
Somewhere, something incredible is waiting to be known.