It's not clear what the requested action is for this issue. I see two possibilities:

1) Add support for reading mrc Images to slicer.
2) Verify and/or Document the usage of ITK_AUTOLOAD_PATH with slicer

For 1, when slicer is build with ITKv4, Slicer the mrc ImageIO will automatically be registered and it'll just work. The following should likely be done, when MRCImageIO moves out of ITK review:
1a) Add the .mrc, and .rec file extensions to list of supported volumes in the GUI, the Image IO, does correctly report the supported extensions, so this may be able to be adjusted for the general case by querying the ITK ImageIOs.
1b) Update the Wiki Page

For 2, It would be nice if there was a configuration in the Application Settings for this environment variable. Previously I used a wrapper script to automatically set this variable specifically for Slicer. On OSX the .app, are more challenging to get this working. As this path should be specific for the Slicer version, the preferences is a logic place for is.

With the adoption of ITKv4, Slicer can load "mrc" files.

It's not clear to me what this issue is for... perhaps to just add it to the list of files now? or it could be considered resolved.

I updated the documentation, it is indeed possible to read mrc files as volumes:
http://www.slicer.org/slicerWiki/index.php/Documentation/Nightly/SlicerApplication/SupportedDataFormat

However, it is not possible to "save" with MRC file format as of now.

Fixed in r22210

Can happily read and write MRC files.