Hi,
That is right. It is not very clear.
On the "DOM" tab, when you select a tag, you can go the the "Attrs" sub-tab and then modify attribute values by double-clicking on the value.
You can also modify css attributes the same way when selecting a css rule in the dom tree.
This is very limited so far. I plan to add live editing of the rule in the "source" sub-tab in a future release.