In Intellij 13 the layout has changed, the Settings button can only be find in File -> Settings and not in the toolbars, and from there you follow the same steps: Editor -> Appearance -> Show line numbers, or search for Line numbers in the Settings search input.

In Intellij 13 the layout has changed, the Settings button can only be found in File -> Settings and not in the toolbars, and from there you follow the same steps: Editor -> Appearance -> Show line numbers, or search for Line numbers in the Settings search input.