Connecting an external ADC to Mega 2560
D[7:0] should be connected to any entire 8-bit port. This will allow you to read the entire conversion result at a time without having to perform wacky bit manipulations.

nCS, nWR, nRD, and nCSTART should be connected to pins in ports A, B, C, D, E, F, or G since those pins need to be individually twiddled and those ports are the only ones that work with SBI/CBI.

You likely want to connect nINT to a INTx pin for interrupt-driven conversion, otherwise any pin should be fine.

CLK should be connected to OCnx if you want to use an external clock (see CLKSEL in CR0).

The rest is just hooking up the references and analog input, and following the timing diagrams in the datasheet.