To use the grow effect, create an object in the DETAIL editor that you wish to have 'extrude' over time. Create a spline path as normal in the detail editor. This will be the path the object extrudes along during the animation.
GROUP (not join) the object with the PATH AS THE PARENT. If you do not make the path the parent, it will not work. To do this, select the path, then hold shift and select the object, then select group. Save your GROUP for loading in the animation.
Finally, add the effect in the ACTION editor for the grouped object.