Flex layout modes are not (fully) natively supported in IE yet. IE10 implements the "tween" version of the spec which is not fully recent, but still works.
https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Flexible_boxes
This CSS-Tricks article has some advice on cross-browser use of flexbox (including IE): http://css-tricks.com/using-flexbox/
edit: after a bit more research, IE10 flexbox layout mode implemented current to the March 2012 W3C draft spec: http://www.w3.org/TR/2012/WD-css3-flexbox-20120322/
The most current draft is a year or so more recent: http://dev.w3.org/csswg/css-flexbox/