I have a file User.DBF, User.MDX but don't know how to open it to modify...

I have some research about the file extension.. but the result return is "Database IV"...

I try to use Visual Basic 6.0 to connect and open it to add new column but fail....

Code:
Dim foldername As String
foldername = "C:\Documents and Settings\Programmer\Desktop\Data"

Conn.Open "Provider=Microsoft.Jet.OLEDB.4.0;Data Source= " & foldername & ";Extended Properties=dBase IV;"

Conn.Execute ("ALTER TABLE User ADD TotalAccess char(255)")
Run-time error
Code:
Operation not supported on the table that contains data.