Assuming you are trying to do swells to a midi string part...

Using the Piano Roll... The violin notes are in the top portion of the pic. You enter these manually or play them in by using a midi keyboard/guitar etc.. You can move the notes around by dragging them up or down to the desired pitch. You make the notes longer or shorter by dragging either end right or left.

The bottom area shows the 'swell' you are looking to achieve. You 'pencil in' the type of volume curve you want to happen for each note, phrase etc. You do this by selecting 'Control' in the View/Edit area. Then in the next drop-down, select either 7 Main Volume MSB or 11 Expression MSB: You can set Velocity here also (how aggressively or softly the player, picks, plucks, or bows a note)