#define DirName "C:\Our App"

[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={#DirName}
OutputDir=userdocs:Inno Setup Examples Output

[Code]
procedure InitializeWizard;
var
  InstallDir: string;
begin
  InstallDir := '{#DirName}';
  MsgBox(InstallDir, mbInformation, MB_OK);
end;