How to enter first order derivatives in a system of 2 PDEs?

Hi, I'm trying to enter the following system in the PDE tool gui:

d2u1/dy2=k*du2/dx

h(x)*du1/dx+g(x)*u1=0

But I'm not sure I'm referring to the first order partial derivative terms correctly, ux(2) and ux(1) don't seem to work as I get a 'Index exceeds matrix dimensions' error message.

I'm entering the following parameters:

c11=0 -1, c12=0, c21=0, c22=0 a11=0, a12=0, a21=g(x), a22=0 f1=k*ux(2) and f2=-ux(1)*h(x)

without success, any idea where I'm going wrong? I find the Matlab documentation very confusing for something so simple, a few examples a little more different from the standard equations would have helped. Also, I'm assuming it's not a problem with solver paramters since these are linear equations.

Thank you!

XL