Tooln is right though, buckwheat really likes warm soil so it may not germinate right away. If we get some sunny warm days, your soil should heat up and it will grow. I live in SE MN and I'll probably be throwing in some buckwheat in a week or two.
What are your goals with the buckwheat? Are you going to let it go to seed and go for a late summer second crop of buckwheat or will you plow it up and try for a fall plot with something else?
Last year I planted buckwheat in late June and I disked it up before most of it went to seed - my goal was to use buckwheat to add organic matter, then plow up in late summer and plant some type of throw and go mix. Everything went well until I planted the throw and go mix and it didn't rain for a couple weeks. It did finally start growing, but it was too late to provide much attraction for the deer.