Why am I getting the error "Unknown data property" when the field exists?
If it is a custom field that you have defined in your module (using hook_field_info()) then you need to add a property_type to your field info.  See https://www.drupal.org/docs/7/api/entity-api/property-info-for-fields for an example and https://www.drupal.org/node/905580 for a full list of possible types.

If your field is a complex (multiple value) data structure you may also additionally need to include property_callbacks as well.  See http://d7.thecarneyeffect.co.uk/describe-field-properties-make-custom-fields-work-entity-metadata-wrappers for a good example of this.