Below are some steps you would need to do:
- Go to model Configuration Parameter and select Data Import/Export pane on the left
- Untick the Single simulation output and click on Apply
- Double click the "Scope" block at which the signals are connected, right click and select Configuration Properties, go to Logging and select Log data to the workspace and select Structure with Time and click on Apply
- Run the simulation
- Double click "powergui" block and select FFT Analysis from the Tools tab
- Now you should be able to select the signals from the list
