At the farm. In the barn? That's a pretty big "storage shed" too. And apparently that's where they put it until DM thought it needed to be moved after hearing about AJ's call to CS. Then it got moved outdoors to a stand of trees.
MOO
ETA: The video from the hangar neighbour that night shows them towing it TO the hangar attached to TB's truck according to the opening statement.
Not trying to be argumentative here, just not certain we have all the facts.
As the previous poster said, we know it was delivered to the hangar. That was in July 2012.
We also know WM was actively involved in things at that time, and would DM seriously have wanted WM asking questions about the incinerator? And the cost? All this is highly problematic. So do we have any reason to think the Eliminator remained at the hangar until May of 2013?
I don't rule out the possibility. There could be a locked (large, to accommodate the trailer) storeroom in that hangar that could have concealed the ghastly thing. But, the risk of discovery would have been great. And then the need for explanations. DM had friends trooping through. And other employees, till after November 2012.
Still possible, but the more people who knew about it, the riskier the whole thing, and the needed web of lies, became.
So, at some point it went to the farm. As Kamille pointed out, it was definitely there after the crime, presumably on the morning of the 7th, and was put in the barn until it was moved to the trees.
Looking closely at the pictures of the barn, that doesn't appear to have been an adequate long-term storage space. It's large, but open to the elements, with many boards and pieces missing, hay everywhere, very dirty. Also, not secure so it could have been stolen from that barn if left there for any length of time.
Enterprising thieves cruise rural areas looking for stuff to steal. A neighbour lost a tractor left out in his back 40; my lawn tractor, which was in an unlocked shed in the rear acreage, was stolen in broad daylight: this on a busy road with nosy neighbours (bless' em). DM would hardly have left such an expensive device in an unprotected location for a long period of time.
So yeah, maybe it was at the farm for a time, but all winter 2012-13? The exposure to wet, mold, ice etc could have ruined it. They could have wrapped it in plastic sheeting but that wouldn't have solved the potential theft problem. Nor the potential for rodents to chew through wiring. Old barns are full of rats and other vermin.
Obviously SS did know something, and if he is called as a witness, may be able to testify to where it was. The reference to a storage area was a possible clue.
Or, would the whole thing, trailer and all, have fit inside any of the big trailers DM had at his disposal? That would have been a possibility.
Did one of DM's "friends" give it a temporary home (horrible thought -- for what?)
Does anyone else recall anything specific to where the incinerator was between summer 2012 and spring 2013?
We all have theories, but what actual facts do we have to go on besides the known facts about the delivery, SS's correspondence with the dealer, and the activities of May 6 and beyond?
I can't think of anything, but I could have missed it.
I have mixed feelings about that. Good because the jury will get to actually see DM and MS operating the incinerator at the hangar in the middle of the night, the same night Tim went missing... but bad, for the same reason. 
