I may be doing something wrong myself. I just tried this morning and I actually got to declare riichi and won by tsumo but it was a bit confusing. Normally though I keep getting to a point where I think I can win but I get this "No multiplier" pop up window. Are you having the same trouble? I just got to where I could declare tsumo and I got the "No multiplier" when I tried. It could be a glitch. I'd try a different game, and just refer to the same rules on that page if they are helpful.
EDIT: Hoo-boy, I've started down the rabbit hole pretty far with this one. I ended up watching part 1 and half of part 2 of this almost 4 hour set of videos on how to play Riichi Mahjongg.
I recommend it just because this guy is completely awesome. He demonstrates really professionally, but uses SD anime figures as stand-ins for players and shows clips from anime. Not to mention the games he uses for demonstration.
I haven't confirmed this but it's possible the "No multiplier" I got above meant that I did not have a "yaku" for an open hand so was unable to win. I also realized at this time I don't own a single mahjongg game.