Well you need to add a default value setting and then add some code to set the value of the control to the default value, you don't need to worry about the member stuff, just using the shortcuts should give you the right value.

So you'll need to add

 [Attributes.Setting("DefaultValue", description = "Enter a default value", control = "Umbraco.Forms.Core.FieldSetting.TextField")]
 public string DefaultValue { get; set; }

And this is a snippet from the default dropdownlist where it checks the default value and adds the list items

                    string dv = string.Empty;

                    if (DefaultValue != null && DefaultValue != string.Empty)

                        dv = StringHelper.ParsePlaceHolders(HttpContext.Current, DefaultValue);

                    foreach (var prevalue in this.AssociatedField.PreValueSource.Type.GetPreValues(this.AssociatedField))

                    {

                        ListItem li = new ListItem(Umbraco.Forms.Data.DictionaryHelper.GetText(prevalue.Value), prevalue.Id.ToString());

                        if (dv != string.Empty){

                            if (li.Text == dv)

                                li.Selected = true;

                        }

                        dd.Items.Add(li);

                    }