Using the Snareline-Manual-Lite, write one measure of 16th notes, and a release on measure 2. Next highlight the passage and go to [b]Play>Transform Live Playback[/b], or [b]Cntrl+Shift+Alt+L[/b]. For a Crescendo click the [b]Crescenco/Diminuendo[/b]radio button. Choose numeric values for the [b]Start at [/b] and [b]End at [/b] velicoties by clicking the radio buttons to the right of the boxes. Lets start at a velocity of 66, and end at 110 then click OK. ";[i]Sibelius will scale the velocities of the intervening notes to produce a crescendo[/i]";. For a Decrescendo, start at 110 and end at 66.

The same can be done by using the default velocities (dynamic markings) with no need to manually set the Live Playback velocities. The defaults are as follows:

[b]ppp=15
pp=30
p=50
mp=60
mf=75
f=90
ff=105
fff=119[/b]

Repeat the same process, but this time insert the dynamic marking ";mp"; on the first note and ";f"; on the last note. Go back to the [b]Velocity[/b] tab in the [b]Transform Live Playback[/b] window. Click the [b]Crescenco/Diminuendo[/b]radio button and the b]Start at [/b] and [b]End at [/b] buttons for ";start at first note's/last note's velocity";...And there you have it.

note: This method does not adjust the volume as the [b]Crescenco/Diminuendo[/b] plugin does.

Sorry guys, the default settings in my previous post were incorrect.
[b]ppp=20
pp=3
p=61
mp=71
mf=98
f=98
ff=113
fff=127[/b]

These can be found in the dictionary under the play tab.

So.....basically what you told us was live playback 101.  I won't speak for everyone, but good effort on the post, though a bit late in the game.