Look at Alpha Industries ( now Skyworks , hxxp://www.alphaind.com/, x=t) search under appnotes for the VCO applications, they have examples for varactor simulation.
Maybe this can help you..
look in the ADS documentation for the SDD component. There is as example for a varactor.. moreover try to look also for an example named MOS_VCO
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You may regard a common MOSFET as a varactor, connect the MOSFET drain, source and bulk together(for example, as B), then this two ports(the gate and B) device is a varactor. Simulate it with S-parameter, you can get its C-V and Q-V characteristics.
ADS example "MOS_VCO_prj" has a design "biased_FET_cap_test.dsn", showed the simulation method.