How to add a deathcount to the new scoreboards in Minecraft PE

You could accomplish this with two command blocks and a comparator. First, put a command block down where they would spawn that says:

testfor @p[x=(x-coord),y=(y-coord),z=(z-coord),r=1] and make it repeating and always active.
Where,
(x-coord) = x coordinate of where they spawn
(y-coord) = y coordinate of where they spawn
(z-coord) = z coordinate of where they spawn

This will detect when they spawn. Next, put a comparator from the testfor command block into the second block that says:
execute @p(put the coordinates of the where they spawn here) scoreboard players add @p 1

Now, whenever they step on top of the command block it will add one score to Death, or whatever you want the objective to be named. Just be careful if the circuit is within render distance of the spawn. I suggest a few blocks underneath the spawn. This system should work, as long as they spawn on the coordinates.