First you need to obtain a copy of the source for the mode. You can download the latest release from the Google Code page for the mode. Or, you can use the git to stay on the bleeding edge:
$ git clone http://git.erlware.org/erlware-mode.git
Once you have the source on your local system, you need to modify your .emacs file to load the mode:
(setq load-path (cons "<path-to-mode>/erlware-mode" load-path))
(require 'erlang-start)
For XEmacs users, you modify .xemacs/init.el:
(add-to-list 'load-path "<path-to-mode>/erlware-mode")
(require 'erlang-start)
If your XEmacs installation already has the default Erlang mode installed, you will probably need to put the erlware mode directory at the front of the path:
(push "<path-to-mode>/erlware-mode" load-path)
Now if you start your editor and load an Erlang file (or create an empy .erl file), you will see an Erlang menu in the menubar.
The Emacs mode supports displaying the Erlang man pages. To do this, the Erlang man pages must be available in uncompressed form. Also, the mode will include any man pages it finds, even if they have nothing to do with Erlang, so the Erlang man pages must be installed by themselves.
If you do not already have the Erlang man pages installed this way, you can obtain a copy of them on the erlware.org download area in files like:
http://www.erlang.org/download/otp_doc_man_R11B-5.tar.gz
You should choose the version of the man pages that corresponds to the version of Erlang/OTP you are using, and untar the file wherever you choose.
Once you have the man pages installed, you must tell the Emacs mode where they are. You can add this line to your startup file:
(setq erlang-man-root-dir "/path/to/man-pages")
The path should be the directory containing the 'man' directory, with no trailing slash.
If you need help installing the mode, or have questions about using it, you can contact the Erlware Google group erlware-questions.