http://search.msn.com/ is MSN's current search engine. It is powered by Yahoo.
http://www.searchmsn.com/ looks like a PPC search engine and I doubt is is related to MSN or Microsoft.
http://techpreview.search.msn.com/ is MSN's "new" search engine. It is currently in testing and is not ready for prime time. To get into their new SE right now, you will just have to be picked up by one of their spiders. Don't worry about getting in yet or ranking well; it's all going to change.