Many of Galaxy S II buyers who got their Mac OSx upgraded to the latest one (Lion), reported that Kies is not compatible with it.
Actually I was facing same problem and I got to know a way to fix this.
To tackle this, you can install an older version and then update it to the latest version. I have tested it works for this version: KiesMac_18.104.22.16855_7. Install it and then go to “Preference”->”Update” and update to the latest version
This a guranteed way to make Kies work on Mac OS Lion.