You do not need a subscription to view another user’s shared campaign but you do need to log in. The log in process is exactly the same as all users.
If you have loaded the notebook from a shared campaign link, it should open the shared campaign. Depending on the access roles created by the campaign owner, you may either see the shared campaign objects or be asked for an access key, see Accessing another user’s shared campaign for more details.
If you are having troubles logging in or seeing the campaign, please check Why isn’t the notebook page loading?