When you use one click to install C5 the script does not change a core setting in the C5 directory, to get the final part to function you need to do the following via FTP. Go to the root of the installation wherever in the file structure you put the install and then follow bellow…
Root > concrete > libraries > 3rdparty > htmldiff.py, open properties and change manual from 0644 to 0700, save done.
Re-load the installation page at your domain and fill in the rest of the data as per the e-mail from Dreamhost.
Hope it helps someone.