Ditto, I have exactly the same problem. (It is working in Pro Tools 8.03 LE however) The AU component is present in my Components folder.
I checked Logic 9.1's AU Manager and discovered that ARP2600 V2 was unchecked and failed validation with the following message,
validating Audio Unit ARP2600 V2 Efx by Arturia:
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
AU Validation Tool
Version: 1.2.3a1
Copyright 2003-2007, Apple, Inc. All Rights Reserved.
Specify -h (-help) for command options
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
--------------------------------------------------
VALIDATING AUDIO UNIT: 'aufx' - 'arpY' - 'Artu'
--------------------------------------------------
ERROR: Cannot get Component's Name strings
ERROR: Error from retrieving Component Version: 4294967246
* * FAIL
--------------------------------------------------
TESTING OPEN TIMES:
FATAL ERROR: didn't find the component
validation result: couldn't be opened
updating properties of AU ARP2600 V2 Efx by Arturia...FAILED!
FAILED!
John