You are passing pointers (Complex*) when your function takes references (const Complex&). A reference and a pointer are entirely different things. When a function expects a reference argument, you need to pass it the object directly. The reference only means that the object is not copied.
To get an object to pass to your function, you would need to dereference your pointers:
Complex::distanta(*firstComplexNumber, *secondComplexNumber);
Or get your function to take pointer arguments.
However, I wouldn't really suggest either of the above solutions. Since you don't need dynamic allocation here (and you are leaking memory because you don't delete what you have newed), you're better off not using pointers in the first place:
Complex firstComplexNumber(81, 93);
Complex secondComplexNumber(31, 19);
Complex::distanta(firstComplexNumber, secondComplexNumber);