1. I think the Spectrum Analyzer block expects a discrete-time signal... So, you can either change the simulation mode of your model from the powergui block, or use a Rate Transition block on the sensed signals to get them sampled. Of course, you need to make sure your sampling rate is high enough.
2. Another thing people usually do with SimPowerSystems models is use the FFT Analysis tool in the powergui block. This will give you an FFT for those particular simulation results.
To do this, you should measure the current (or voltage) in a Scope block, turn on Structure with Time logging in the Scope, and then pull up that FFT Analysis tool from powergui.
3. I almost forgot, there is also an Impedance Measurement block in SimPowerSystems which can be coupled with the Impedance vs. Frequency Measurement option in powergui. You basically connect this block across the terminals you care about, and then you get an impedance over a specified frequency sweep.
Check that out and see if it gives you the right frequency peak(s).