To select an envelope, select it from the tree list at the bottom of the editor. Envelopes that belong to the current piece are shown on at the root level of the tree, while envelopes that belong to a library are shown in their respective groups. A default library is always available in newly created pieces. This default library contains a few commonly used envelopes: "attack", "double attack", "triple attack" envelopes are somewhat similar to percussive instruments, while "smooth" and "default" are more like soft flute or violin: they raise and decrease slowly. The "pulse" envelope is like a "smooth" envelope, but its decomposition in many little spikes allows creating a vibrato like-effect that can be used to alter the timbre locally, enhancing and softening the harmonics in short periods.
You can hear how an envelope sounds like by selecting it and pressing the "Play" button in the editor.
When you select an envelope, its name and shape appear in the editor above. Envelopes that belong to a library are not editable. To edit them, you must open the piece file that defines the library.