If after installation a user does not have access to the Email Marketing application, it can be resolved by the administrator creating a specific permission set and applying it to that user (or any others that cannot see the Email Marketing App)
Login as Salesforce Administrator and navigate to Permission Sets, and create a new one.
Enter Label, API Name and a description. Click Save
Select the Assigned Apps and click Edit
Click Edit and then add ddm.Email Marketing to Enabled Apps
Apply the new permission set to any users that do not have the app available (as well as the relevant user permission)