Hook up a Bound node to the geo you want to lattice. To this, add an Edit node - this will be the node you pull points about with. Create a Lattice node and plug the geo into the First tab, the bound into the 2nd and the Edit into the 3rd.
Might be an idea to copy the divisions of the bound into the Lattice's divisions.
*further note*
since the lattice is linked to the bounds of your object, it isn't ideal for dynamically changing objects - eg a fluid sim. In the case of this, you're better off creating a static lattice object that will deal with the shape of your object throughout it's simulation. I think.
