The PAM pulse generator block can be found under Default/Transmitters library/Pulse Generators/Electrical. Double clicking on the PAM generator brings up the properties. The value of bits per symbol, should be 2 bits for 4 level PAM. After that in the same window select the checkbox “User-Defined PAM map”. Clicking the icon beside PAM Amplitudes will open up the MxN parameter Properties window that you see in the picture attached. In that window you can define the levels of your PAM signal.