Salesforce custom object synced but values not syncing.

I've created a custom object in Salesforce and synced it to Marketo. The custom object is now showing in the schema but the values for the custom object are not coming through to the contact.  Any thoughts on why that data is not syncing?  the custom object is a MDR to contact and the record includes the email address for mapping.  But when I look at the record in Marketo, the custom object does not appear in the drop down on the custom object tab.