Actually, it's less complicated than you probably think.
The box that appears, simply shows you all objects that have conditions/events/actions, depending on what you are trying to add.
Once you choose one of those (double-click), it shows you all available conditions/events/actions for that exact object.
After choosing one of those, more often than not, you have to type in additional parameters.
The whole dialogue system always provides you with additional information on the currently selected item on the bottom of the selection window.