Your mech battalion will be in your items under chests.
But before you open it, be aware that it will be a massive drain on your food supply.
If you are fairly new, don't open it until you are producing a very large excess of food.
If you are still in beginner protection don't open it.
If you can't train transporters, don't open it.
A mech battalion could eat 100k food an hour or more.