Make sure the photo is in process 2012(current) under camera calibration in Develop. The dehaze won't show up for images in an older process version. Also, the slider shows up in the very unintuitive location of the "Effects" tab, not under basic.
Check the /help/about dialog to see that LR is running in CC mode, not 6.x mode.
Every time my LR asks me to confirm my subscription (closer to every 7 days than every 30), it stays in 6.x mode after the login process. I then need to restart LR CC to get it back into CC mode with the dehaze controls being available.