To use Altiverb 6 in Logic 9, please make sure you run Logic in 32 bits mode (hit get info on the Logic app to set this).
Then open from the logic preferences the Audio Units manager.
Hit the first check box in the Altiverb line, TWO TIMES. :)
This will force Logic to load the plug-in. Logic will put the plug-in in the incompatible plug-ins menu, but Altiverb does fully function.