vhdl code for alter megacore functions

Do you want the code that Quartus II uses to synthesize the megafunction or do you want the simulation model so you can simulate your design? If the latter, then look under eda/sim_lib/ for the basic models. For more complex megafunctions, you can often direct the Megafunction Wizard to generate a Verilog or VHDL simulation model for your megafunction instance. The Quartus II Handbook is a good starting point.