Your not alone, I also had this problem. When one isn't intimately familiar with and practiced in handling the pantographs, its hard to know for sure, where to push/pull and how hard.

In the 'stored' position, the internal servo has the monofilament released fully, and the pantograph is pushed down and clips into place behind/under two small tabs at the base of the pantograph. That holds it down in storage.
(one should always raise the pantos for storage, and then manually push down and under the tabs to lock them, don't store them with the servo having pulled the filament down to lower the pantos).

So to release them, the not-grab-from-the-sides, is to prevent the pantos being deformed by pressing in on them from the sides (I think).
You need to look and find the two tabs under which a cross bar of the panto is captured. Them push or pull the panto along the axis (toward the nose end) of the loco to get that cross bar out from under the tabs.

Thank you both so much for the replies. I feel like I have a slightly better idea of what is going on but somehow still can't do it. When I first got the locomotive I had put it on the track before finding the manual -- is it possible that raising or retracting the pantograph caused problems? See below for my attempts in case that helps.

I actually found some videos that show people unlocking it but although it is hard to see it doesn't seem like mine is exactly the same. (e.g. the one below at approximately 6:00)

trimmed.mov (18,935kb) downloaded 10 time(s).

Aargh, sorry, I gave the wrong number somehow. The actual locomotive I purchased was a Marklin 39126 - Class E 10.12.

Success! Thank you so much!