Use the Sine Wave block to generate two sinusoids, one at 15 Hz and the other at 40 Hz. Use the Matrix Sum block to add the sinusoids point-by-point to generate the compound sinusoid:

Transform this sinusoid into the frequency domain using an FFT block. See the ex_fft_tut model:

The scope shows peaks at 15 and 40 Hz, as expected. You have now transformed two sinusoidal signals from the time domain to the frequency domain.

You can use a Spectrum Analyzer block in place of the sequence of FFT, Complex to Magnitude-Angle, MATLAB Function, and Array Plot blocks. The Spectrum Analyzer computes the magnitude FFT and shifts the FFT internally. See the ex_time_freq_sa model:

The blocks in the Power Spectrum Estimation library compute the FFT internally.