Download the trial version of Interarchy from http://nolobe.com/interarchy/
Extract the app, and move to the Applications folder.
Open Interarchy, and you will see a window like this:
In the protocol drop-down, select WebDAV
In the server, type the IP or hostname you use to connect to your DNS-345
You can either leave the port as default (80) or change it to 443
Type in the username and password you have set for webdav access.
In remote path type: /webdav/YOUR WEBDAV SHARE
(Where YOUR WEBDAV SHARE is the name/path of the folder you have set as a web-dav share)
This will connect you to your DNS-345 via webdav.