After you have created a capacity, you can put it into use by linking the capacity to tickets. To do this, go to Settings → Tickets → Price keys in the Admin.
Do you still need to create a capacity? Then look here.
Do you still need to create a price key? Then look here.
Select the price key that you want to link to the capacity by opening an Event and the accompanying and relevant Performance and Sales period. You can open each level by clicking on it. Select the price key by clicking 'Edit'. Select the correct capacity under 'Capacities'.
Click 'Save'. The capacity is now set.