Put the two files somewhere in Emacs' loadpath. It varies by system, but you can display yours by entering emacs and asking for help on the variable load-path (C-h i). The directory ~/.emacs.d is often on the load-path for many systems, and if it isn't, you can add it by inserting the following line early in your .emacs file:
(setq load-path (cons "~/.emacs.d" load-path))
Now, add the two lines below to your .emacs file:
('require dommod-mode)
('require dommap-mode)
Opening any file with the ending .dm or .map should active the correct mode; although you might have to turn on font lock manually (again, depending on your emacs setup): M-x font-lock-fontify-buffer
Alternatively, and easier if you just want to try them out to see if they're useful, you can load them manually. M-x load-file and specify either dommap-mode.el or dommod-mode.el; then open a .map or .dm file.
P.S. A really good resource on Emacs is the
EmacsWiki
Re: Emacs modes for mods and maps
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