If lack of money is the one thing making you miserable then yes more money will make you happy. In all other circumstances it won't miraculously make you cheerful.
I'd love a Lamborghini and a massive house, but I don't think it would change my actual level of happiness. My wife and kids, good friends, and a job I enjoy are what provide me with that.
I suppose it depends on the person. I know that a Lamborghini will make me happy. Plus having more money might make it easier to have more time for family and friends and it will make it easier to pursue hobbies as well.
For all the rich people who are not happy because they have a lot of money. I have a simple solution. Give your money to me.
EDIT: I think for a lot of people who are not happy even when they have a lot of money is because they probably don't know how to spend it in a way that will aid their happiness