It is possible that the plug-in has not been scanned correctly.
To force a re-scan of the plug-in, do the following:
Go to Live Menu > Preferences. Click on the File/Folder Tab.
Under Plug-in Sources, make sure that "Use VST Plug-in System Folders" and "Use Audio Units" are both on. The latter if you are using Mac.
On Windows, add
Program Files\Steinberg\Vstplugins or your VST plug-in directory to the list of paths.
The plug-in should now show up in the list.