In this case, there's no need to instantiate the object... it exists in memory. Much like the aqFile or aqString runtime objects. Actually, a better equivalent would be to take a look at what's in the AQAScriptExtensions.tcx file as that should have some similar items.
You can also take a look at the ExcelCompare extension available at
http://smartbear.com/support/viewarticle/9046/ as that should show you how to properties and such.
Basically, what you're building in the Description.xml is a description of the object with methods and properties declared in the XML. Your code unit would then contain the methods described in the XML file (a function called "set", one called "get", your "excel" method, etc).