[Chevalier]

Ah, I think it's a UI problem. Open up your friend's name. In the bottom left corner is a little green chat icon, which signifies that you're sending a Hangouts message. Tap that and you should see the option to send a text to the associated phone number.

It's not necessarily intuitive unless you know to look for it, and it might not work if your Google Contacts are incomplete (as in, you don't associate your Hangouts friend with his phone number). But it works really well for me.

[icelancer]

That doesn't work. I search for Bob Robot. I see Bob Saget (wtf) and other celebrities named Bob. OK that's not helpful.

Oh finally, Bob Robot comes up. I click it. It says "Bob Robot isn't on Hangouts right now..." Well I don't want that. I want to send an SMS.

OK let's look for a green chat icon. Nope, just a smile button to put emojis in there.

This is OSX's Chrome plugin as stated.