Before I tell you how to create a mortar, I must say that in the latest versions of I&A, mortars cannot be created due to wide-spread abuse (mainly people sitting in base killing everything and leaving nothing for the other 39 players to do). I can guide you through a code edit if you'd like to re-enable them on your server. Regardless, below is how you create a mortar in ArmA 3.
To create a mortar, there are two bags (as you've found): Folded Mk6 Mortar Bipod and Static Mortar Bag. Currently, when equipping the bags, their icons are invisible in your gear menu, but they are there (hover over and you'll see the correct tooltip).
Get either the Folded Mk6 Mortar Bipod backpack or the Static Mortar Bag backpack
Find where you want to place your mortar and drop the bag on the ground
Get the backpack you didn't get last time
Walk up to the bag on the floor, scroll and select something alone the lines of Assemble Mk6 Mortar.
Voila! A la mortar.
Hope this helped some