There are several ways to open the Score Editor in Logic Pro.
Click the Score button at the bottom of the Arrange window (or use the Toggle Score Editor key command, default assignment: N).
Double-click a MIDI region.
Note: In order for this option to work, you need to choose the setting “Double-clicking a MIDI region opens: Score” in the Logic Pro > Preferences > General > Editing pane.
Choose Window > Score (or use the Open Score Editor key command, default assignment: Command-3).
Drag the Score button, which changes to the Score Editor window as you drag it.
Hold down the Option key and double-click any event in the Piano Roll Editor.