First problem: QGraphicsScene does not have a slot called addEllipse(), that's a standard member function.
Second problem: You specify the types of arguments, for both SIGNAL and SLOT, in the connect, not the actual values. Connect is establishing a communication path not actually placing the call.

You need to write slot to receive the valueChanged(int) signal and connect to that. The code of that slot can then fetch the spin box value and do whatever it likes with it, including calling addElipse().