I just ran into an issue were I am no longer able to select my keyframes. In short..
I have tried everything offered at this link with the same results listed above....
I am assuming this is a option that I am missing with in after effects. When I revert back to an early project I have none of these issues. So at some point today I have done something that I can't not resolve.
You have time-stretched the layer and the keyframes end up at inaccesible temporal locations, because they do not coincide with discrete frames. Old issue, that should have been fixed in CS4. Are you running the proper 9.0.2 update? If not, install the update. it may fix the issue. If not, undo the time stretch...