There was a typo in bullet 5 of the original Assignment 3. It used to say:
5. Identify an activty or set of activities in your use case and create a sequence diagram that shows how objects in the system handle those activities.
It now says:
5. Identify an activity or set of activities in your activity diagram and create a sequence diagram that shows how objects in the system behave when those activities are active. Note: you need to pick a set of activities that allows you to create a non-trivial sequence diagram. (I don't want to see a sequence diagram with just two objects and a single call between them!) Be sure to indicate what activities the sequence diagram is covering.
The point is that in step 4, you are asked to create an activity diagram that models the use case that you created. Now select a subset of those activities to model in the sequence diagram. Just be sure to pick a large enough “chunk” to ensure that you can create an interesting sequence diagram.
Let me know if you have any questions.