First, please note that darcs comes with a very good HTML documentation, which is available under the /usr/share/doc/darcs-*/manual dir. That's the place where everything is properly documented, not the manpage. Using darcs [subcommand] -h is usable only as a reference, too.

If you're completely new to darcs, then start at /usr/share/doc/darcs-*/manual/node4.html.

Please also note that in order for the darcs send command to work properly, you must properly configure your mail transport agent to relay outgoing mail. For example, if you are using postfix, you need to edit /etc/postfix/main.cf, see the Using a relay host part of the postfix package documentation for more info.