Ok. here you go.
First step you need to get the Node you want to add your child node, then add your child nodes to this node.
Since you want to Append your child to this
Designs Id="D100"
node, use the following code to get this node first.
XmlNamespaceManager namespaces = new XmlNamespaceManager(doc.NameTable);
namespaces.AddNamespace("flp", "http://www.w3.org/2001/XMLSchema");
XmlNode nextNode = doc.SelectSingleNode("/flp:Tab/flp:Designs", namespaces);
To create each element you should append the namespace element
flp
in order to match with other elements, to add namespace to each Element use the following code.
XmlElement D100 = doc.CreateElement("flp","D100","http://www.w3.org/2001/XMLSchema");
D100.SetAttribute("Number", "2");
XmlElement Code = doc.CreateElement("flp", "Code", "http://www.w3.org/2001/XMLSchema");
Code.InnerText = "B";
D100.AppendChild(Code);
Please note that you need to re-order the element to you add to the D100 element and add its child nodes like the other node.
finally add the newly created D100 node to the Designs child node
nextNode.AppendChild(D100);