A wonderful/happy place that is unique to each person. Each person has their own "Merryland" aka their own "happy place."
Ex 1: If you've ever seen "His and Her Circumstances," you will know that the character Asaba created this term. Asaba's Merryland is a place where he is surrounded by girls since he loves them so much.
Ex 2: "My Merryland is a place where I'm with the one I love"
Ex 3: "My Merryland is a place where I'm surrounded by piles and piles of money"
Ex 4: "My Merryland is a place where I can eat as much chocolate as I want"