To my knowledge, BTS 2006 R2 needs WSS 2.0 so that the configuration later can be done more smoothly. After which, you need to extend a virtual server in the Sharepoint site. With that, then your BTS can be installed and configured….
As for your MOSS, you need to install the BizTalk Window Sharepoint Service Adapter to your MOSS server also. If not, your WSS adapter will not be able to work. To install, you need to run the BizTalk installer CD but when you reach the Feature choosing part, you choose only Additional Component->Window Sharepoint Service Adapter and uncheck everything else.