Data tab on Model Options dialog.
You can use the scope control to switch between 'whole building' and 'single zone' modes. To model a single-zone within a building, make sure you are editing the zone you want to model and change the scope to 'Zone'.
If you want to include building self-shading effects in a single-zone analysis you should select the 'Single zone + shading' option. Note that if the single zone is part of a large complex building, simulations using this option are often no quicker to complete than the full building simulation. This is because the shading calculations required for the large number of shading elements used to model the rest of the building are slow.