There is a waveport tutorial in Ansoft OTS explaining this.
A few advanced remarks on CPW ports:
1) By default the waveport outerboundaries are perfect E. Do not make your port too high or too wide, otherwise you may excite different modes (waveguide modes). Try limiting yourself to lambda/2.
2) In very high frequencies where you also simulate the actual 3D via configuration that shorts the grounds, and have very strong radiation above the trace - do the following: Open the Setup1 options, click the advanced tab and check the use Absorbing Boundary Conditions on ports (ABC). Make sure that the port will cut two vias in the middle (that short the left ground to the bottom groundplane and the right ground to the bottom ground plane). Draw the port such that it is as wide as the vias below the trace and as about half lambda wide above the trace.
Explanation: When selecting the ABC checkbox the port boundaries absorb the radiation (default is a Perfect-E box). ABC is more accurate, but without shorting the 3 grounds together you will end up with 4 nets in the port (left ground,trace,right ground,bottom ground). The waveport first mode may not be the CPW mode. Therefore you need the vias crossing the port and shorting the grounds (now you're back with two nets in the port and a CPW excitation).
3) In both cases use the ZPI impedance calculation in which the arrow defines just the +/- sign of the field (and not the Z0 calculation, so it does not really matter to which ground it is pointing). Be consistent with all ports in the model. For example the line should always start with the trace and end with the ground. If you fail to be consistent then ang_deg(S12) will be wrong by 180degrees (+/- sign).
Itai