EntityMetadataWrapper comes from the Entity API module; by default it only knows about core field types, you have to explicitly tell it how to handle custom fields.
That involves adding
property_type and possibly
property_callbacks keys to your field info array.
See http://drupal.stackexchange.com/questions/34823/what-does-the-property-type-key-do-in-hook-field-infoWhat does the 'property_type' key do in hook_field_info()? for a bit more information, and here for a code sample.