You need to pass the X and Y as references (if you’re not sure what that means, read about it here) so the function can set them, like so:
float X;
float Y;
MyController->GetMousePosition(X, Y);
// Now your X and Y have their corresponding values
Edit:
I just noticed they have an example in the documentation:
https://docs.unrealengine.com/latest/INT/API/Runtime/Engine/GameFramework/APlayerController/GetMousePosition/index.html