I added a custom field mapping, but I don’t see the values copied over to the appropriate system yet.

Custom field mappings will only be effective for records that have been modified after the custom field mapping was added. A re-sync will also allow the new field mapping to appear. 

To re-sync your WorkflowMax clients and contacts to HubSpot, visit the integration settings page at https://hubspot.workflowmax.com/my-settings and locate the Sync Settings tab. Find the option labeled "Sync WorkflowMax/XPM clients and contacts to HubSpot companies and contacts" and look for a date picker labeled "Sync records modified on or after." Roll that date back to, say, one year ago; this will tell the integration to resend those records from WorkflowMax/XPM to HubSpot. Save your settings, and then the updated information should begin to appear in HubSpot within 10 to 15 minutes.

To re-sync your HubSpot companies and contacts to WorkflowMax/XPM, send a note to support and they will re-sync all records with the new mapping. Optionally, and assuming there are company/contact properties you are okay with changing en masse, you can perform a bulk edit in HubSpot, which will mark the records as recently modified and trigger a re-sync.