First, put mairix.el in your Emacs search path
and put (require 'mairix) into your
.emacs file. Then, use
M-x customize-group mairix RET to set your preferences
for mairix.el. The most important items are Mairix File
Path, Mairix Search File and Mairix Mail
Program. The latter specifies which mail program should be
used to display the mairix search results. Currently, RMail, Gnus
with mbox files, and VM are supported. If you use Gnus with
maildir or mh, use the native Gnus back end nnmairix instead.
If you use another Emacs mail program which is not yet supported by mairix.el, it is pretty easy to integrate it. See Extending mairix.el, on how to integrate it into mairix.el.
Now you should be ready to go. See Using mairix.el, for the available commands.