I have tried creating a new template by copying the EntitySpaces_C#_Custom_Entity.csgen template to a new file.  However, the new file will not compile even though I haven't change a single line of code! 

How do I go about creating new - working - templates?

Here are the instructions David gave me:

These are the steps I use to copy a MyGeneration 1.2.0.7 template to be used as the starting point for a new template. It is all done within MyGeneration. It's not as simple as having a "Copy" button, but it's pretty painless.

• Open the template you want to copy.
• Open the Template Properties panel. (Click on the collapsible splitter on the left of the window, or click on the Properties icon, to the right of the "As" diskette icon at the top of the window.)
• Click the New Guid button. Each template must have a Unique Id.
• Change the Title.
• Change the Namespace. Do not use the EntitySpaces namespace.
• Click File -> Save As (or the "As" diskette icon at the top of the window.)
• Change the "File name".
• Change the "Save in" location.
    (The MyGeneration Template Browser uses the namespace to organize
     things. The Save location of the template can be anywhere you
     want beneath the MyGeneration\Templates folder, but you will
     find maintenance much easier if you create a Windows folder
     structure that matches your namespace structure.)
• Click "Save".
• You are now ready to replace the Template/Interface Code with your own.
  

Thanks Trevor!

That worked beautifully.