I don't think to. A monostay bike has a single large diameter tube where it joins the seat tube.
What, exactly, are you trying to accomplish?
I want to install a rear rack. I'd prefer not to have a seatpost attached rack and wonder what my options are...
Does your bike have threaded rear dropout eyelets? If so, buy a conventional rear rack and bolt the rack's legs to the eyelets and attach the rack's upper mounting brackets to the seat stays using P-clamps. If not, you can use P-clamps at both the bottom and top of the seatstays.
I've used a mono-stay adapter myself but it doesn't look as though you need one since you have two separate seat stays to attach p-clips to. Mono-stay adapters come in handy when the seat stays are in the shape of a wish bone. I've also used them around the seat post of a really compact hybrid frame where the stays were so far below the rack that I couldn't reach them with the mounting hardware.
Most ordinary rear racks will work for you. The adaptor you mention is for another type configuation than the type you have. Some bikes have one tube from the seat tube to the rear brake that splits into two there (thus the 'wishbone' name you see there), yours doesn't do that.
Biking isn't a sport because anybody can do it. I can bike, you can bike. For goodness sakes, my mother can bike! You don't see her on the cover of Sports Illustrated, do you?
1) That bike has no eyelets for rack attachment
2) P-clamps to the seatsay is preferable to 3
3) Monostay adapter attached to seatpost,
Mes compaingnons cui j'amoie et cui j'aim,... Me di, chanson.