MELPA is actually just a repository of emacs packages. The emacs package manager has been included in emacs since version 24. For 23.3 you first need to get a compatible version of package.el
(there is one here).
After putting package.el
in your load-path
, you then need to add MELPA to the list of repositories:
(require 'package)
;; Any add to list for package-archives (to add marmalade or melpa) goes here
(add-to-list 'package-archives
'("MELPA" .
"http://melpa.org/packages/"))
(package-initialize)
From the EmacsWiki.
To then install OmniSharp, first refresh the package archive with M-x package-refresh-contents
, then you can use M-x package-install
RET omnisharp
.