To create a new pattern, double-click an empty cell.
This opens the Pattern editor where you can enter events to create a new pattern.
To edit an existing pattern, double-click the cell.
This opens the Pattern editor where you can edit the events of the pattern.
Note
You can only edit patterns in the Pattern editor, not styles. If you want to edit your styles this way, you must first convert the part of the style to a MIDI pattern.