Using PE10 and am trying to set opacity for Video 2 but the add/remove keyframe button is missing for Video 2. It is there for Audio 2 and Audio and Video for 1 and 3.
If at any time you do not find the add/remove keyframe button on the track, you can just select the clip and click on properties button below the monitor.
In the properties panel you can easily(and with more power) add/remove keyframes using the Show Keyframes and toggle animation buttons.
I noticed the missing add/remove keyframe button many times on many different video tracks, not just video 2. Re-launching helps sometimes. If I have LOTS of video tracks, if I scroll down, mess with other tracks and scroll back up, the add/remove keyframe buttons might appear. I hope they fix this! FYI: Here's a photo of the missing add/remove keyframe buttons on video 2.
When I encounter any sort of GUI anomaly, I check that my video driver is the most recent from nVidia (in my case), as PrE interfaces heavily with the video driver, and one of the first signs that there are problems will be with the GUI display.