Hello Usman,
You can use the DisableCalc.exe utility, you will find it in the installation folder, or in the menu Start > Exlevel > Disable calculation.
Click Yes, and then restart 3dsMax. After starting 3dsMax, it will ask you that calculations are off, and if you want to turn it on, click No.
To enable calculations, you need to restart 3dsMax again, and answer Yes to the same question.
Yes, GrowFX can calculate even if Manual update is enabled, for example when rendering. Apparently forestpack uses the render function to get the resulting mesh.
Thanks!