After you have created a composite structure (by using convenient drag n' drop to create part Properties) you may go back to the matching "architectural" class diagram used for planning the connections and generate composition associations (if you wish) by dragging the part Properties out of the context Class (i.e. they become "roles", Property ends of composing Associations from the context Class).
TIP: I recommend that you avoid trying to "compose" complex systems using associations; prefer the structure diagram "systems-engineering" approach wherever possible ! Note also that you can't show connections in a purely classifier-level diagram, so the associative composition diagram only shows only a subset of the structural information anyway.
