As taken from :
http://wordpress.org/extend/plugins/media-custom-fields/faq/
How does this plugin handle my data?
Media items are stored in
WordPress just like regular posts. This enables us to use native
WordPress functions to store and retrieve your custom fields. Due to
this, your data is stored in a very future-proof way, in the postmeta
table of the WordPress database.
So you would do it the same way you would do it for a post or page using get_post_meta, but you would pass the media attachment ID in as the post ID