In order for Particle Flocker to affect a particle node, it must first be assigned to one. This is the standard behaviour for all Maya field nodes.

In Maya the normal method of assigning a particle node to a field is to select both objects, selecting the FX Menu Set and then choosing the Fields/Solvers > Assign to Selected menu item as shown below:

Particle Flocker provides a shortcut to this menu item in its Utilities menu which does exactly the same thing. You may find it quicker to use this option rather than having to browse through the Maya menu system: