When you first bring your rabbit home allow a couple of days for them to get used to their cage and accustomed to their surrounds.
This will also give you an opportunity to see where they decide to go to the toilet so you are able to set up their litter box in a spot of their choosing.
Allowing your rabbit to run unsupervised is not a good idea, especially when they are new to your home.
1. Remember to keep your eye on your rabbit for all out of cage time, if they pee or poo out of their litter box, do not yell or get angry with them, just soak or pick it up with a tissue and place the feces or urine and rabbit back into their litter box immediately, they will catch on soon enough.
2. Do not rush your rabbit, allow therm to explore and go about their business and learn to use their litter box in their own time, like people all rabbits are different, some will catch on faster than others.