Hello Cyriaque!
Unfortunately the Finale guys have forgotten to implement the possibility to change the buffer size in Finale 2009 and 2010 on Mac. They promised to implement it in a future version. I recommend leaving the buffer slider of VE at x2.
Crackling playback may also have different reasons. Maybe your hard-drive is not fast enough. If you have a firewire audio interface and a firewire hardrive, they are in each others way. It may also help, to install the latest version of drivers for your audio interface. Have you tried it with the built-in audio? If the playback is clean then, the problem has something to do with your audio-interface.
Best regards,
Andi
Vienna Symphonic Library