If I understand correctly you tried to define the linkage directly from transaction SWE2. The event first must be defined as a starting event to the task or workflow you what to trigger. remember that you also need to transfer objects/container elements in the workflow binding in order for your task/workflow to work.
Go to your task/workflow (transaction PFTC or SWDD) and write the event as a starting event. when you set the event as active this will automatically write the linkage, there is no need to so so directly.
