Segue Editor is called from the playlist: right-click a track whose crossfade parameters you want to edit, and select Segue Editor (to edit 2 tracks) or Segue Editor 3 (to edit 3 tracks).
A window opens:
Example: Segue Editor for 2 tracks
The red line shows the track volume level. Use the mouse to move tracks and square points.
Custom crossfade parameters are saved in the playlist. When you open and close RadioBOSS, the parameters will remain. To save the playlist to a file in RadioBOSS top menus click File -> Save (or Save As...).