I've just retested this and the gtk2-engines-aurora package is installed as part of the ubuntu-satanic package:
...and the .so file is part of the gtk2-engines-aurora package:
sudo apt-get install ubuntu-satanic
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
gtk2-engines-aurora satanic-gnome-themes satanic-wallpapers
The following NEW packages will be installed
gtk2-engines-aurora satanic-gnome-themes satanic-wallpapers ubuntu-satanic
0 upgraded, 4 newly installed, 0 to remove and 46 not upgraded.
Need to get 17.7MB/17.8MB of archives.
After this operation, 18.4MB of additional disk space will be used.
Do you want to continue [Y/n]?
That's on 32-bit Maverick. So I'm a bit baffled as to what happened. All I can suggest is try installing the ubuntu-satanic meta package again and see if it also installs the gtk2-engines-aurora package. Maybe do it from the command line (as above) and send me the output if you still have problems.
dpkg -L gtk2-engines-aurora | grep libaurora.so
Sorry about this!