On the middle-left part of the screen are the lists of all the instruments and expressions (representing the arpeggios and pitches):
The list icons
The top-left icons concerns the list below. In the screenshot above, the first icon is selected, showing the instruments (Beep being the only instrument).
The second icon, when clicked, opens the arpeggio list:
The third opens the pitch list:
By double-clicking on any item, or pressing enter, the related page opens.
Click the 
icon at the bottom to add a new item.
By right-clicking on an item, a contextual menu opens, with more options:
You can also drag’n’drop the items to change their order, the song will be automatically remapped with the new one. You can also multiple-select with shift or ctrl.
The right icons
At the top-right are several icons, which open panels not here, but on the larger area on the screen (where the Pattern Viewer is when starting AT):
The icons open respectively;
- The pattern viewer (or use F2).
- The instrument editor (or use F3).
- The arpeggio editor (or use F4).
- The pitch editor (or use F5).
Note that opening these last panels automatically opens their aforementioned list panel too. Handy!