Add the following line to your .emacs:

(require 'eldav)

If you want to use a proxy server, the following setting
is also needed:

(setq eldav-proxy "http://your.proxy.server:8080")

Then you can access WebDAV files by specifying a magic
file name like:

:http://your.webdav.server/path/to/file/name

(Add preceding ':' to the WebDAV URL.)

NOTICE: You need the MULE version of Emacs!