The small example file in the installation section shows you how to define some new commands to run the macros, and how you can bind some keys to these newly created commands. You can load this file on demand (just use "ESC-x load < return > < name - of - file >"), or you can append it to your s:mg-startup file, which MG automatically reads upon startup.
MG compiles your current buffer. The file in this buffer must end in ``tex''.
The same options for starting the server are available than for ShowDVI: supply no format name, a format name or '?' as a format name. It works exactly the same way.
Note that the server is called fully asynchronously, thus you are still able to stay in your favourite editor while the server is compiling. If you queue several requests (don't do this!), i.e. you 'tex-compile' while the server is still running and busy with another file, you will get a short message in the status line when the server finally accepts your request.