Click on the "Properties" tab. You'll then see a list of all the properties you've created. Click on the name of the property you'd like to add the units to. Then click on the grey "View Units" button. Click on the blue "Add a New Unit" button. You'll be able to add a unit name and property address. To add more details (monthly fees, number of rooms etc), you'll find the instructions you need in this help doc.