Provided that your plug-in XML and Dll files are properly located in the Wwise bin/plugins folder, and that the CanBeInsertOnBusses and CanBeInsertOnAudioObjects field in the XML plug-in description (PluginInfo/PlatformSupport section), it may happen in some situations that the plug-in is not listed in the list of effect.
The main cause of this problem is often related to a security update for Visual Studio 2005 SP1 that Microsoft recently deployed. Amongst other things, this update changed the Visual C Runtime DLL from version 8.0.50727.762 to 8.0.50727.4053. After this update, everything compiled that uses the runtime dynamically linked, gets their dependencies updated to the new runtime. If you don't have this update then the Wwise plug-in Dll will not load (hence you don't see it in Wwise) because it cannot find one library that it depends on.
See http://support.microsoft.com/kb/971090/ for more details.
The solution to this problem is to download and install the update (see link below) and then recompile the plug-in.
