Volume and Expression
On the Edit screen, you'll see a setting labeled Volume, and another setting labeled Expression. These 2 settings combined determine the volume for the channel.
To set these values initially, you should set the loudest volume needed for the plugin. For example, if the cello section has a melodic line that needs to be louder than all other instruments, playback this melody with Expression set to 127, and increase Volume until the melody is at the desired level. Then, reduce Expression until the cellos are at a mix volume (ie, balanced with all other instruments).
For the duration of the piece, increase or decrease the Expression value as needed for cello crescendos or decrescendos respectively. Do not change the Volume setting (except if the entire cello part needs to be raised or lowered to balance its mix volume. Then subsequently do not change Volume).