You may check following:
- After registering the event dll in Tridion Content Manager config file, you might have not re-started the SDL Tridion Content Manager COM+ application - If so, restart the COM+ application and try again
- You have not specified the SDL Tridion version - if it is SDL Tridion 2009 ensure you have enabled the corresponding event in the SDL Tridion Configuration Manager
- The absolute path of the event dll that has been registered in the config is wrong - if so, provide correct absolute path of the event dll
- The user with which your Tridion service is running do not have permission on the folder which contains the assembly - provide the user running Tridion Service permission over the folder which contains event dll
It would be great you can share the snippets from your Tridion Content Manager Configuration file and the code that you are using along with the version of SDL Tridion - this will help the community provide you more helpful suggestions.