Click on the profile of the person you would like to send the message to. That's where the message option is. There are some bizarre Houzz rules on who you may or may not PM. Something like you both have to be following each other, unless the recipient has enabled the 'allow all messages' feature.

Thanks!

Mad Gallica,

Well, I've tried clicking on the person's profile and I'm simply not seeing a field to where a message can be sent.