When you first start Capo, you will be greeted by an invitation to drop a song on the application. You can drag songs onto this window from Finder, or directly from iTunes.
Once your song is loaded, you will be presented with the main Capo interface:
On the left side of the window you'll find the song's cover art, information about the song, and the Capo heads-up display (HUD). The HUD contains the current position in the song, and controls to adjust playback speed and pitch.
On the bottom of the window, you'll find a volume slider, playback controls, and the zoom slider.
The timeline indicates the current position of the song, and contains the song markers. You can also set up your loops in this area.
The spectrogram display shows the currently-playing audio, and is where you will enter notes.
The notation area, directly below the spectrogram, is where you'll find the chord entries that you place in the document, and the note entries that correspond to the where you place notes in the spectrogram area.