I'm pretty sure that the HW simulator won't handle buses wider than 16 bits without crashing. If you want to experiment with more complicated logic, I'd suggest checking out

Logisim. (Note that Logisim is limited to 32-bit buses.)

That said, assuming that you had an Add16 that had

INPUT

a[16], b[16], c;

OUTPUT

out[16], carry;

you would want Add64's PARTS to be

Add16(a=a[0..15], b=b[0..15], c=false, out=out[0..15], carry=c0);

Add16(a=a[16..31], b=b[16..31], c=c0, out=out[16..31], carry=c1);

Add16(a=a[32..47], b=b[32..47], c=c1, out=out[32..47], carry=c2);

Add16(a=a[48..63], b=b[48..63], c=c2, out=out[48..63]);

--Mark