Here is the code for older versions of Acrobat:

/*add New to the File menu*/
app.addSubMenu({ cName: "New", cParent: "File", nPos: 0 })
/*add US Letter to the New submenu and create a new page*/
app.addMenuItem({ cName: "US Letter", cParent: "New", cExec:
"var d = app.newDoc();"});