After you add a Box Office Staff user to your site you will need to also assign them to the venues.
Screencast/video covering this: http://screencast.com/t/2WfOrRyAi
Step 1: Go to Settings > Box Office
Step 2: Associate
No worries - you can also do this from Manage > Venues > Click the venue > On Box Office Staff List click "manage"
Now the staff member can see all of the events that are linked to just the venues they have access to.