Добавьте название процедуры созданной автоматом в procedure InitializeWizard();
Пример
Код:

[Setup]
AppName=My Program
AppVerName=My Program v 1.5
DefaultDirName={pf}\My Program
OutputDir=.
Compression=lzma/ultra
InternalCompressLevel=ultra
SolidCompression=yes
[Languages]
Name: rus; MessagesFile: compiler:Languages\Russian.isl
[Code]
{ RedesignWizardFormBegin } // Don't remove this line!
// Don't modify this section. It is generated automatically.
var
NewButton1: TNewButton;
procedure RedesignWizardForm;
begin
{ NewButton1 }
NewButton1 := TNewButton.Create(WizardForm);
with NewButton1 do
begin
Parent := WizardForm.WelcomePage;
Left := ScaleX(288);
Top := ScaleY(200);
Width := ScaleX(75);
Height := ScaleY(25);
Caption := 'NewButton1';
TabOrder := 2;
end;
{ ReservationBegin }
// This part is for you. Add your specialized code here.
{ ReservationEnd }
end;
// Don't modify this section. It is generated automatically.
{ RedesignWizardFormEnd } // Don't remove this line!
procedure InitializeWizard();
begin
RedesignWizardForm;
end;