The reading is an exceptional one. 豆 is okay, and is the on'yomi reading from the kanji. The 納 part is weird though. Instead of being のう it's なっ. But, if you can remember the meaning of this word, you can just think about how you do not (なっとう) want to eat natto. At least a lot of people don't want to eat it. It's quite the acquired taste.
Don’t get me wrong. I love Japanese food, but I could never just eat natto alone.