The full MEI 2013 release is available from here. Besides that, the following materials are provided separately:
The following RelaxNG schemata files may be used for validating XML against the MEI format. These files also contain further restrictions expressed in Schematron. It is recommended to use an XML editor capable of validating against these additional rules as well.
The documentation of the 2013 release, containing Guidelines, a tag library, and further documentaiton of the technical module structure of current MEI, is available as a PDF.