You will need to extract them from whatever kind of zip they are in and then, cut the character's folder and paste in the Mugen Folder named 'CHARS' Then you need to open the data folder and open f with a text editor. But please put in a bit of effort to look before requesting away. If you can't find a specific character you can request for it in the request board. There are several Portal sites that keep a database like The Mugen Fighter's Guild Database. Searching with Google will turn up a lot fast. To acquire more playable content you will need to go to someone's site that creates content for MUGEN or create it yourself. The engine comes with one character Kung Fu Man and it also comes with the default screenpack and lifebars. How many characters and/or stages does M.U.G.E.N come with? Get the latest version of mugen (1.0) right here.
The latest version of MUGEN is available on Elecbyte's site. More information can be found on its Wikipedia page found here M.U.G.E.N (also known simply as MUGEN) is a freeware 2D fighting game engine designed by Elecbyte.