To Reproduce:
- Add GridViewCalculatorColumn to a RadGridView.
- When you begin edit a cell the value is not selected.

Workaround:
Create a custom RadCalculatorEditor and then change the editor in EditorRequired event:
void radGridView1_EditorRequired(object sender, EditorRequiredEventArgs e)
{
    if (e.EditorType.Name == "RadCalculatorEditor")
    {
        e.Editor = new CustomCalculatorEditor();
    }
}

public class CustomCalculatorEditor : RadCalculatorEditor
{
    public override void BeginEdit()
    {
        base.BeginEdit();
        RadCalculatorDropDownElement editorElement = this.EditorElement as RadCalculatorDropDownElement;
        editorElement.EditorContentElement.TextBoxItem.SelectAll();
    }
}