SQLDataSource Question

Hello,

I'm having a problem with SQLDataSource . I'm running a stored procedure that needs to have a the param "@param1" but I have no idea how to supply it with SQLDataSource. Here is what I have so far:

Here is the message I get when I try to view the page:

Procedure or Function 'list_sp' expects parameter '@param1',
which was not supplied.

Front End:
            <asp:sqldatasource id="CustomersSource"
            OnUpdating="CustomersSource_Updating"
            SelectCommandType="StoredProcedure"
            connectionstring="<%$ ConnectionStrings:DatabaseTEST%>"  
            runat="server" SelectCommand="list_sp">
                <UpdateParameters>
                    <asp:Parameter Name="Param1" Type="String" />
                </UpdateParameters>
            </asp:sqldatasource>

Back End:

    protected void CustomersSource_Updating(object sender, SqlDataSourceCommandEventArgs e)
    {
        e.Command.Parameters["@Param1"].Value = BaseUtility.regionString(this.User.Region.ToString());
        e.Command.Parameters.Add(e.Command.Parameters["@Param1"].Value);
    }

Needless to say, my solution doesn't work and I really don't know what I'm doing as SQLDataSource is new to me. Does anyone out there know how to supply the param value to my stored procedure? Thank you very much :)