DataGroomr caches a list of fields of Salesforce objects, in order to refresh a list of fields for all objects, go to Matching Models and hit Refresh Objects.
In order to refresh fields of a specific object, edit a dataset and hit Refresh Fields on the Fields tab.
Note, you have to be DataGroomr's Account Admin to be able to refresh metadata and we recommend being Salesforce administrator as well to make sure all fields are available.