Moving mesh.

I don't know how to put some conditions in the moving mesh when I set up surface wave.
I found an example file whose named 'Sloshing tank' through comsol model library.
It is pretty similar with what I'm doing at the moment.
so, I've got an expression which is ' u*nx+v*ny ' in prescribed mesh velocity of Sloshing tank.
But when I had used it on my model, error sign has been up.
Please, help me :]
I hope you will give me lots of advices about moving mesh - prescribed mesh velocity.