One doesn't. Outlook can't open newsgroups at all (unless you use an
add-in, usually commercial, or you connect to an Exchange server that
presents the newsgroups as folders). Outlook calls Outlook Express to read
news and Outlook Express out-of-the-box can't handle yEnc. You can,
however, get tools like yProxy (
http://www.brawnylads.com/yproxy) that can
fit between OE and the newsgroup. You can also get a newsreader like Xnews
(
http://xnews.newsguy.com/) or Gravity
(
http://lightning.prohosting.com/~tbates/gravity/) that can handle yEnc
natively.