Volume modulation (also called Expression) is key for most instruments, as it allows for expressing feeling. Click on the VOL icon on the toolbar to toggle its display. It appears as a magenta line/curve over the cells, as shown here:
By default, no pitch modulation is defined (so the orange line appears fully horizontal, in the middle). You can define meend curves across one more cells by right-clicking on one and selecting Set Meend option. You will then see the following Set Meend panel:
Here you can tailor your volume curve by adding, removing or moving points. A click on an empty area of the graph will add a point. A click on a point, or dragging a rectangle around one or more points will select them. You can then drag them to another position or press the delete key to delete them.
To save your changes, click on APPLY.
To remove any pitch modulation, click on CLEAR.
To leave without saving the changes, click on CANCEL