[plt-scheme] if without third argument

On Fri, Jun 27, 2008 at 06:48:50AM -0500, Robby Findler wrote:
> Because we found that it helped us catch our own mistakes more easily.
> If you want, there is "when" and "unless" that are like the two
> argument if, or you can use r6rs.

Can you please tell me how I can "use r6rs"?  Is this something I can
put in my .mzscheme file, so it will automatically get called whenever
I start up mzscheme?

Thanks,

Tamas