How to feed external clock generated by PLL into SOPC system?

When you click on finish on the wizard, it is then asked to you if you want to add the Quartus II IP file to your project, answer yes.

Then in your project you have a vhd or v file and a symbol file.

If you use schematic, insert a symbol (Edit menu or icon) and on the libraries the symbol of your pll will appear, so just add it. Else if you use only vhd or v file, instantiate the pll as a classical component (look at the file inserted in your project for the entity). For example, without modifying the parameters you have something like that :

ENTITY pll IS
    PORT
    (
        areset        : IN STD_LOGIC  := '0';
        inclk0        : IN STD_LOGIC  := '0';
        c0            : OUT STD_LOGIC ;
        locked        : OUT STD_LOGIC 
    );
END pll;