Since this word is made up of a single kanji, it should use the kun'yomi reading. When learning the kanji, you didn't learn that reading, so here's a mnemonic to help you with this word: You're having this crazy dream. The dream allows you to do anything you want. Every time you ask "hey, can I do this?" The answer is always "you may" (ゆめ). Think of some things you'd want to do if you could do anything. Ask permission and get this answer in your imagination.