If you are running Adobe CS5 or CS5.5 and have updated to OS X Lion, you may encounter an error with the Adobe Extensions Manager utility in which you cannot install any new extensions.
This issue was found in early December by a number of Adobe CS5 users who had upgraded to OS X Lion, and after investigating the issue, Adobe issued a patch to the software that should allow the Extensions Manager to work properly again. The update has been available in Adobe's Update Manager, so if you regularly update your Adobe software, then you should be fine if you have not yet upgraded to Lion and plan on doing so; however, if not, then you can either update with Adobe's software or do it manually.
Adobe offers the update as a standalone downloader that is available here:
To install the update, download the updater for your version of the Adobe Extensions manager, open it to extract the DMG installer disk image located in it, and then open the disk image file. Then quit the Adobe Extensions Manager utility if you have it running and apply the patch in the disk image, following the onscreen instructions to install the patch.
Be sure the Extensions Manager is fully quit before installing, and you may try rebooting into Safe Mode (hold the Shift key at startup) to ensure no Adobe products are active if you run into problems applying the patch. After installing the patch, your version of Adobe Extensions Manager should be either 5.0.2 or 5.5.2.