The testing program Microsoft runs is aimed more at developers than end users. People who are fairly adept at tracking down and isolating program faults and generating feedback that's useful to developers. Also people who will need to ensure that none of the changes made will affect any of their programs, and give them time to get fixes in place if it should happen any are required.
The other thing you need as a tester, is an understanding that the software is not going to be perfect, and there will likely be some problems associated with using it. You need to be willing to suffer through these problems.
Generally, Microsoft is only interested in testers that are developers for Fortune 100 companies. They're not interested in Joe Average who just wants to try something new with their computer. If you want to do that, install Linux on your system. You can gain access to up to the minute versions of most software if you want. Development of software is done in a much more open fashion, where anyone who wants to poke and prod it is welcome to do so, along with anyone who wants to take it for a test spin. You'll have to compile the source yourself, but it's not that difficult after the first 5-6 times.