In some cases the app may start up with the photo camera icon active instead of the Annotate button.
It is true that in some cases the Annotate button is not immediately visible. For now, you can circumvent the problem by closing the app and restarting it, then you will see the button at the bottom of the screen, to the right of the taskbar (icon of a pen on the screen).
We are aware of this problem and will update the app as soon as possible to fix it.