The simple answer is to either upgrade libatk or find an older version of MySQL Workbench. Sometimes there are newer packages in the backports repository, but I checked karmic-backports in Ubuntu's Package Repository and a newer version has not been backported. Ubuntu 10.04+ does have a newer version of libatk that's compatible with MySQL Workbench. You could try installing on 9.10 ignoring dependencies and hope it works anyways.
dpkg --ignore-depends=libatk1.0 -i mysql-workbench-gpl-5.2.33b-1ubu1004-i386.deb
The only other option is to download the source from 10.04 Maverick and recompile it on Karmic. Get the source from http://packages.ubuntu.com/source/maverick/atk1.0. You will need the .dsc, .diff.gz, and .tar.gz file from that page.
sudo apt-get install dpkg-dev
dpkg-source -x atk1.0_1.32.0-0ubuntu1.dsc
Once it's done building, you will have a number of .deb files in the parent directory. You will need to install any over what's already installed.