fix buoyancy

Hi everyone,

A very simple question.

I am trying to implement buoyancy in my simulations and I started learning the fix buoyancy from the example in the command

fix bu all buoyancy level 0.06 dim z density 700

(http://www.cfdem.com/media/DEM/docu/fix_buoyancy.html)

Nevertheless, when I have it on I get this error

ERROR: Invalid fix style: "buoyancy"

any suggestion?

Cheers,
Luca