Mochi dough is meant to be very sticky! In fact, the stickier it is, the softer your final mochi will be :). If you're at the mixing stage, get a bowl scraper or spoon, cover it in oil and scrape the sides of the bowl. After this, you may find that your dough comes together! If not, microwave the dough until more of the water evaporates and your mochi isn't as runny.