Modifying crit range and modifiers on drag-and-drop monsters

Hey, Unless I am looking in the wrong place, it doesn't seem like I can modify weapons/attacks on the monsters. Is that right? I can do it just fine on player characters.

You can, its just really complicated.  the crit range of a d20 roll is 1d20cs>X where X is usually 20.  To make a npc sheet do a different crit range you are going to need to get the template it normally spits out and manually make a token action using that template format.  There is a philosophical debate to be had about why a Dm would want to add a crit range to npcs as its almost always a mistake being made alongside a DM vs player mentality but theres always those rare occasions when its for a reason that isn't that.  I suggest you rethink why you are doing that.  Just give the creature a better to hit, theres no need to give it a higher chance of critting as that extremely skews the DPR of your encounter and would lead to deaths.  

The NPC sheet lacks that ability, because NPCs in 5e do not use an expanded crit range mechanic (There might be an exception, but I can't think of any offhand). However, you might find some workarounds in this thread .