Looking through your sample data, this could be a little tricky.
I plotted the data and it looks like the jump takes place before the greatest magnitude of down force and that the time in the air is actually when the force plate has the highest positive values.
I would suggest using the max() function to find that value and then the find function to find all data points greater than the max - 10. The number of points found would be the time in the air, based on the instructions you mentioned in your post.
myMax = max(data(:,5));
vals = find(data(:,5) > (myMax - 10));
pointsInAir = numel(vals);
If you know how much time each data point represents, you can then calculate air time.