Looks promising 
Probably who ever wrote the instructions forgot to check their compass …
The photo from your last post seems to show a type of valve which would require a key, have a quick look in a shed for a T looking metal peace with a square opening on the end, something like this (link).
If you don’t find it, no worries, you can simply use a pair of pliers or a wrench if you can get it in there to turn the valve. Only a quarter turn should be needed to turn on the water fully. Good news is that after it is ON, you can probably leave it on indefinitely as your main irrigation valves should be rated for a continuous pressure exposure.
In case everything works and you wish to consider leaving the valve ON all the time, we will need to have a discussion about the backflow preventers as I am unsure if your system has one. This is a topic for another time.
As far as the other question in your last post, the valve finding solutions out there, as far as I know, work on the types of valves you already have control over, meaning your irrigation valves, not a dumb water valve. To find such valve one would either put a high voltage on the wire and detect EM fields or ‘chatter’ the valve (turn it on and off quickly) in order to make an clicking noise you could look for. I guess you could get a metal detector, but valves today have less and less metal in them, preferring plastics instead.