Due to API limitations, Constant Contact does not support syncing tags at this time.
Constant Contact response here.
However, here's the workaround to help you sync the Tags.
Most of the existing Constant Contact subscribers just export the records with all tags and import to Salesforce, as a one-time migration, to checkboxes which match the tags.
Tags added to Constant Contact records cannot sync back to Salesforce, so this process would be best managed from the Salesforce side.
This is all manual today using Dataloader.io to sync Tags. The easiest way is to export tags into a CSV weekly or monthly from Constant Contact with the Main List records then import using Dataloader.io.