Kind of broad with your issue.
So is the twixtor plug in just not showing up when you open AE or does it not work?
Have you restarted AE and/or your comp since placing it in the proper folder.
Your plug in should be placed in your "plug ins" folder that comes from your after effects cs4 folder thats located in your applications.
Thats for CS4 or later (could be for CS3 and earlier too, I just only have CS4 and 5).