You need to change the working hours for either the whole project or the specific resources who do work on weekends (as appropriate). This is available in the calendar view. Choose "Calendar View" from the View menu. Then resize the green bubbles to include weekends, or add new green bubbles.
If only some of your resources work on weekends, you can change their specific normal work weeks. If no one normally works on weekends, but there's one weekend where stuff is getting done, you'll want to change the work schedule instead of the normal work week (see drop-down in lower left corner).
You may also find it helpful to turn on "Show Off-hours" (in the View > Gantt Chart menu).
Hope that covers it. If you've still got questions, post 'em here and I'll try again.