Where are you dragging this FROM? If you're not dragging the Curves EFFECT located in the Effects & Presets pane within AE, you're dragging the wrong thing!
I suspect you're trying to drag from the plugins FOLDER on your applications drive, and that is definitely NOT how to do it. I almost never see the .aex suffix, and when I do it's because I'm nosing around on my applications drive.
If you really drag&drop the file from Explorer you're going about it completely the wrong way.
If that's not the problem, you need to explain better and provide the necessary technical info like system specs, comp settings, footage used and so on.
No, I'm dragging it from the Effects and Presets folder, which is why the other effects are able to be used. If i was dragging it from the folder on my Applications drive it wouldn't only not work with one single effect.
For the guy below,
Windows 7 professional.
Intel(R) Core(TM) i7-3770K CPU @ 3.90 GHZ
32 GB of memory
64 bit OS.
PS. I'm a fairly experienced AE user, been using it for about a year now. This problem only first arose when i started working on my new computer.