In your case, you're in essence creating a new class, based on TPOSI. For the sake of argument, let's just say your subclassing the TPOSI class. With the Implements keyword, you have to include in your "subclass" *all* the methods and properties of the superclass; TPOSI. The compiler error is simply saying that you don't have a "SetIMEState" property or method in your subclass. If you were to create one, it's signature would have to exactly match that of the superclass, or you'll get runtime errors. By "signature", I mean the arguments, and argument types. They would have to agree with their counterparts in the SetIMEState method of the superclass. Now you see why I called this approach a hack. If you want a bit of advice, avoid using Implements, unless you have a complete and thorough understanding of the superclass, all it's methods and properties, etc..
Compuware's choice of VB/VBA as the underlying scripting language is what's holding you back. This is one of the reasons we're ditching TP for testing web apps. The company I work for has a "framework" in place for testing desktop apps and we don't want to rewrite everything. For testing web apps, it looks like Ruby and the Watir module. Besides, it's all free and there's a very active development community for Ruby!
Hope that helps.