初学VB调用dll，请帮忙看看。在线等

lz.dll中的文件为：
Public Key As StringPublic Function showmsg(str As String)
       MsgBox str
End Function调用：
Private Declare Function showmsg Lib "D:\vb\dll测试\lz" (ByVal d As String)Private Sub Form_Load()
       Dim str As String
       str = "测试dll"
       showmsg str
End Sub
运行后弹出错误：“找不到lz.dll的入口”
是什么原因