Above the video description, click the "Share" tab.
When that comes up, you can click the "Email" sub-tab.
Type the email address and your personal message and click send.
If that doesn't work for you, you use the first tab that came up ("Share this video") and copy the link into an email message. If the video is from a playlist, it will automatically set to share the video IN the playlist. If you just want that one video, you can uncheck the box that says "share with playlist starting from..."
To make it even easier, you can just copy the url from your address bad and paste it into an email.
As far as sending videos WITHIN YouTube is concerned, that feature seems to be phasing out, but that isn't the fault of Google+! If Google is trying to phase out the YouTube inbox, it is because it's so underused. Most people don't bother to check it, and most youtubers rely more heavily on outside communication like Twitter, Facebook, Instagram, Pinterest and Google+.