The first time I switched the inspector to console mode, it took me a while to realize where I could enter expressions.
Tim mentioned two things that will make the console prompt more noticeable: 1) focus it when switching to console mode, so that a blinking caret will appear; 2) change the separator above it to a wider split view divider.
The new Inspector UI should fix this.
This will be fixed by the patch in bug 17870.
This changed in r31079.