You can change your permissions either via the chmod command from the *nix command line (log in to your account via ssh/telnet), or (my choice!) via most ftp clients.
I use (and love) leechftp - just google for it, it is free.
While I have not yet installed docuwiki on DH, I strongly suspect that, in spite of install instructions to the contrary, on DH you want to set those “writable” directory’s permissions to 755, not 770 as instructed. Assuming you are running PHP as CGI (which most users are, at present), PHP runs as your user, so 755 permissions will be sufficient, and DH often gags at setting looser permissions.
I plan to do a docuwiki test install later this week, I’m just too busy at the moment to try it now, so let us know whether or not you get this worked out. If not, I’ll report back what I discover when I do my test install.