Here are the steps for syncing a lookup field on the SFDC Lead Object.
Step 1. Make the lookup field visible on the SFDC Marketo Profile
Step 2. If the lookup field is referencing a SFDC Custom Object, make sure the SFDC Custom Object is visible to the SFDC Marketo profile by setting the object permissions (Set to read only and view all records)
Step 3. If you want all the values of the field to sync over to Marketo immediately you can have your SFDC admin use the SFDC Data Loader to change the last modified date/system mod stamp for all the records.
Note: It is recommended you confirm with your SFDC Admin that the last modified date/system mod stamp is updated when the value of the lookup field changes.
See this article for part I of syncing a previously hidden SFDC field.
Additional resources for learning about the Marketo integration with SFDC.