myConnection.Open()
    rtb_Address.Clear()
    txt_Name.Clear()
    Dim str As String
    str = \"SELECT * FROM table1 WHERE (cus_ID = \'\" & txt_ID.Text & \"\         
        I would recommend the following:
 Using cmd As New OleDbCommand("SELECT * FROM table1 WHERE cus_ID = @ID", con)
    cmd.Parameters.AddWithValue("@ID", txt_ID.Text)
    dr = cmd.ExecuteReader()
    While dr.Read()
      rtb_Address.Text = dr("cus_Addr").ToString
      txt_Name.Text = dr("cus_Name").ToString
    End While
 End Using
cus_ID is probaly a numeric data type, but you try to query it with a string: (cus_ID = 'thevalue').
Just remove the enclosing ': (cus_ID = thevalue)
or better, use a parameterized query to prevent sql-injection.