Motion can read third-party After Effects-compatible plug-ins in addition to FxPlug plug-ins. When Motion is on a Power PC-based Mac, there is a setting in Motion preferences that allows you to point to a folder where such plug-ins are located on your disk.

If you are running Motion 3 on an Intel-based Mac, you will not see this option in preferences. This is because After Effects-compatible plug-ins are not supported on Intel-based Macs.

Many third-party plug-in developers are now supporting Apple's native FxPlug plug-in format. Contact the makers of your After Effects-compatible plug-ins to inquire if they have an updated FxPlug version.