How to creat dll file and use dll for in VB 6.0?

Hi!
I'm a fortran beginner. Now i'm using DVF 6. And I want to creat a user interface for my for. program by VB6. I have done an example from several references, but It doesn't work.
First, I created a Fortran project as dll project type.
My fortran code is below:
======================
subroutine pro1( a, b, c )

!DEC$ ATTRIBUTES DLLEXPORT :: pro1
!DEC$ ATTRIBUTES ALIAS : 'pro1' :: pro1

integer a,b,c
c=a+b

end subroutine
==========================
Then, I run fortran and a pro1.dll file was created.
after that, I created a vb project. I copied also the dll file into the same directory with the vb project.
code vb:
Private Declare Sub pro1 Lib "pro1.dll" _
(a As interger, b as integer, c As integer)

Private Sub cmdbut_Click()

Dim c As integer

call pro1(3,4,c)
txt1.text=cstr(c)
End Sub
=================
When i run vb program, there was a error: "can not find the entry point pro1 dll".
Could you help me?