もう一回書きます
unit Unit1;

interface

uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
StdCtrls;

type
TForm1 = class(TForm)
Button1: TButton;
Label1: TLabel;
procedure Button1Click(Sender: TObject);
private
{ Private 宣言 }
public
{ Public 宣言 }
function add(a,b:integer) : integer; //ここでadd関数宣言

end;

var
Form1: TForm1;

implementation

{$R *.DFM}
function TForm1.add(a,b:integer):integer
var
c : integer;
begin
c := a + b;
result := c;
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
Button1.caption := inttostr(add(1,2));//inttostrはint型のデータをstrings型に直す関数
end;

end.