This occurs on some systems, if you are first selecting the SS2Instruments folder in the PREFS box and the sounds are not then showing in the Sonik Synth browser then all you need to do is select a different folder (any other folder) and select it. Hit OK in the PREFS box.
Now just go back to PREFS and select the correct SS2Instruments folder.
The sounds should now appear in the browser section of Sonik Synth 2.