If you want to measure the capacitance with respect to the frequency, place an AC generator between the node you want and ground (I suppose you want the capacitance referred to ground). The parameter save current has to be "yes". Then name the node, f.i. "out".
Do the simulation and in the result sheet place an equation to first calculate the inductance as Z=out/out.i (sorry I don't remember the exact synopsis and I've no access to ADS in this moment).
Then place a second equation to calculate the capacitance in F as C=-1/(6.28*imag(Z)*freq) or C=-1e12/(6.28*imag(Z)*freq) if you want pF.