Excel to serializable object
The indendation looks off - assuming it's not a paste glitch with tabs vs spaces, the scope-opening brace should line up with the method's signature:


public myClassFromXsd excelToMyClassFromXsd(String excelFile)
            {



Like this:

public myClassFromXsd excelToMyClassFromXsd(String excelFile)
{


Also, C# type names should be PascalCase, as well as any public members: 


myClassFromXsd should be MyClassFromXsd
serializeXml should be SerializeXml
deserializeXml should be DeserializeXml


The get_Xxxx methods in the COM Interop interface aren't following this convention, but they're COM Interop methods, with their own conventions - they're not an example to follow for typical C# code.

I like that you're wrapping your StreamWriter in a using block. I don't understand why you're not doing the same with the StreamReader in the deserializeXml method - the reader.Close() call could then be removed:

    public myClassFromXsd deserializeXml(String xmlFile)
    {
        myClassFromXsd mcfx = new myClassFromXsd();
        XmlSerializer serializer = new XmlSerializer(typeof(myClassFromXsd));
        using (StreamReader reader = new StreamReader(xmlFile))
        {
            mcfx = (myClassFromXsd)serializer.Deserialize(reader);
        }
        return mcfx;
    }


I would give mcfx a meaningful name: as it stands it looks like it's just a shortened version of the return type's name - I like having a result for these kinds of things. Identifiers should have a pronounceable name that carries their meaning.