In Chrome and Safari/Webkit (without spreadsheet setting enabled) you simply click anywhere beneath a rule or in the empty space to the right and add creates a new line where you can enter a new rule.
In the latest Webkit, with spreadsheet setting enabled to mimic Chrome's functionality, adding a rule by clicking in the empty space no longer works. The only way currently in that scenario to add a property is to click on an existing rule and click tab or return.
It is in the roadmap.
*** This bug has been marked as a duplicate of bug 178022 ***