Problem in Maxplus2

I'm a new learner,when I programing my first programe, I have a problem``` Maxplus2 said that "can't find design file adder"

My programing is simple, it's a fulladder, the programing is:

--*******************************

LIBRARY IEEE;

USE IEEE.STD_LOGIC_1164.ALL;

USE IEEE.STD_LOGIC_ARITH.ALL;

USE IEEE.STD_LOGIC_UNSIGNED.ALL;

--**********************************

ENTITY FULLADDER IS

PORT(

A,B :IN STD_LOGIC_VECTOR(3 DOWNTO 0);

C :INOUT STD_LOGIC_VECTOR(4 DOWNTO 0);

S :OUT STD_LOGIC_VECTOR(3 DOWNTO 0)

);

END FULLADDER;

--**********************************

ARCHITECTURE A OF FULLADDER IS

COMPONENT ADDER

PORT(

A :IN STD_LOGIC;

B :IN STD_LOGIC;

C :IN STD_LOGIC;

CA :OUT STD_LOGIC;

SUM :OUT STD_LOGIC

);

END COMPONENT;

BEGIN

U0:ADDER PORT MAP(A(0),B(0),C(0),C(1),S(0));

U1:ADDER PORT MAP(A(1),B(1),C(1),C(2),S(1));

U2:ADDER PORT MAP(A(2),B(2),C(2),C(3),S(2));

U3:ADDER PORT MAP(A(3),B(3),C(3),C(4),S(3));

C(0)<='0';

END A;

Hoping someone could help me.