If your XLN Audio plug-in is not showing up in Logic Pro X, please follow these steps: Go to the Logic Pro X menu -> Preferences -> Plug-in Manager...
Is the plug-in available in that list? If so, please select it and click Reset and rescan selection and then try loading it again. If it's not there, please try deleting the AU Cache:
- Load the Finder application
- In the top menu, click on Go to bring down the menu options
- Hold down the [alt] button on your keyboard and the Library item will appear in the menu. Click on it.
- Navigate to: Library -> Caches -> AudioUnitCache
- Delete the com.apple.audiounits.cache file
- Restart your computer
- Try loading the plug-in in Logic again