You can get to the keyframe bezier handles by using the Direct Selection Tool, the white arrow tool. Click on any keyframe on the stage, not in the Motion Editor. I don't use the Motion Editor, mostly for the same reasons that you stated. I'm guessing that the intention for this tool is that it mimic the timeline tools in After Effects. It doesn't. The minimal documentation is, at best, misleading. Working with the Motion Editor is sort of like poking a snake with a stick: something will happen, you have no idea what, and that something will probably not be what you anticipated.
Thank you for that.
I discovered that bezier handles are not available for X, Y, Z manipulations in the motion editor (! agast !) but only for things whewre you do not need them, like tint and alpha. Bizarre.
A useful tip for the motion editor is to use the otherwise useless feature of multiple customised interfaces. If you make one called "motion" - or something - and close everything except the motion editor - which you maximise to take the enotre screen - you can then jump to the editor at full scheme. Why the dear,dear Adobe people chose to make this grey on grey is yet another of those mysteries, like the nightmare Microsoft ribbon. Software houses get wedded to the notion of change for its own sake, perhaps: they certainly don't ask the use what they want.
As a further comment on the UI - supposing that Adobe read these things - there is a big problem with the layers in the timelines. The name of the layer is shown in white font (why??) when the layer is selected. Additionally, my usual windows selection colour (light yellow) is superimposed over the whole line. So the name is white on light yellow - unreadable, even if it wasn't in weeny font - and the animation colors are overlayed with a sickly blend of thsi and the underlying blue, etc. A moments thought would say: put an arrow icon (or something) to show which layer is selected, and leave the rest alone.
Flash isa steep cliff to climb, with precious few hand holds. Why Adobe have to make it quite so hard to even see properly is beyondf me.