Please note that this tutorial was written for Logic Pro 9 and although the basic principles are likely to remain the same for future updates, some of the screenshots may not exactly match up with what you see in future versions.
To rescan for Audio Units in Logic:
1. With Logic open, select Logic Pro → Preferences → Audio Units Manager… from the menu bar.
2. Click the 'Reset & Rescan Selection' button, followed by 'Done' once the scan has completed and given you a report.
NOTE: This process should be followed after installing plug-ins/updaters, or moving AU components in the Finder.
Further information on the AU manager & manual plug-in activation:
Logic uses the Apple AU Validation Tool to ensure that only plug-ins that fully comply with the Audio Units specification are used with Logic Pro. This process minimises problems that may occur when using third-party Audio Units. You can choose to manually activate Audio Units that fail the validation test by selecting the checkboxes in the Logic column. However, be warned that these plug-ins can cause problems (in some cases even loss of data), so it is not recommended!
If you decide not to heed the warnings and activate plug-ins with failed validations, and find that it prevents Logic from opening:
1. Hold Control-Shift when opening Logic.
This opens Logic Pro in Audio Units safe mode – only plug-ins that pass the validation test will be available in this mode.