You need to get a Shimano Flight Deck computer and wireless harness. I think it ran about $120 in total when I bought it.

I'm getting rid of it next time replace my bar tape and brake cables. It ends up useless to me because I always end up hitting the buttons when I'm on the hoods, which either puts it on a function I don't want to see or stops recording my ride entirely. Showing gears and virtual cadence is nice, but not worth the hassle. I'll replace it with a Cateye Strada Wireless or Double Wireless (or maybe a garmin edge 500 if I feel like spending money).