There are several possibilities, and some of them are specific to Mac OS X 10.7 (Lion) and Mac OS X 10.8 (Mountain Lion), but the general strategy appears to be to do a "Reset and Rescan Selection", which in some instances includes running Logic Pro 9 in 32-bit mode; doing the "Reset and Rescan Selection"; closing Logic Pro; and then running Logic Pro 9 in 64-bit mode, where once again you do the "Reset and Rescan Selection", as explained in this post:
http://www.absolutemusic.co.uk/community/content/computer-music/logic-pro-scan-a udio-units/
There also is a bit of information in this post from Apple Support:
http://support.apple.com/kb/TS3934
And there is information about "Reset and Rescan Selection" in the Logic Pro 9 User Manual, which you can download from the Apple website in PDF format . . .
Lots of FUN!
P. S. As you probably know, running Logic Pro 9 or any other dual-mode application in 32-bit mode is done via right-clicking on the application and selectiing "Get Info", where you will find a checkbox for "Open in 32-bit mode", which when checked instructs the operating system to run the application only in 32-bit mode. For a dual-mode application, when "Open in 32-bit mode" is unchecked and you are running Mac OS X in 64-bit mode, the application will run as a 64-bit application. As I recall, beginning with Mac OS X 10.7 (Lion), the operating system is 64-bit, but with Mac OS X 10.6 (Snow Leopard) you have the option to run the operating system in 32-bit mode or in 64-bit mode, provided your Mac hardware (processors and so forth) supports 64-bit mode, hence since you are running Mac OS X 10.6 (Snow Leopard), doing the switch from 32-bit mode to 64-bit mode and back might require rebooting to 32-bit mode; doing the 32-bit rescan; rebooting to 64-bit mode; doing the rescan; and then rebooting to 32-bit mode, where you also need to ensure that the "Open in 32-bit mode" checkbox is checked or unchecked as appropriate. It is easier in Mac OS X 10.7 (Lion) and Mac OS X 10.8 (Mountain Lion) . . .