If you are using Edge you have to switch on 'Use Flash Player' via Settings (top right corner) and then Advanced. However it is definitely a bit twitchy about playing Flash.
I've not used this site before but I tried it again with Edge, IE, Opera, Firefox and Chrome and couldn't get it to work despite allowing Flash Player. I noted though that on first switching on the PC this morning Adobe Flash Player put in an update.
Could have been a co-incidence.