I have 2 custom fields in my WordPress system when creating new posts. One is called homethumb
(is the title of an image) and another called description.
I
Here is another option if you want to display anything other than images and videos in your feed: http://wordpress.org/extend/plugins/rss-custom-fields/
All of your custom fields will now be displayed in your feed.
Further options to be able to select which custom fields display in a feed will be available in subsequent releases of the plugin!