Compare button and menu item are operable only when 2 Main Nodes are active. So, when you want to use this option you should have at least 2 scans of Registry in the current file (see. Registry Scan ))
You can choose any key for comparing, when you click the Compare button. If the relevant key exists in another active tree the procedure of comparison will be accomplished, otherwise the following message will be displayed: "Appropriate node does not exist :..".