Install Google Now On Rooted Android ICS Devices

Step 1: Connect your ICS device to your computer.
Step 2: Open Root Explorer, and browse to the /system folder
Step 3: Within the folder, you will see build.prop. Open it in a text editing application such as Text Editor.
Step 4: Make sure ro.build.version.sdk = 16, after which, you’ll need to hit "Save."
Step 5: Browse to /system/app and rename GoogleQuickSearchBox.apk to GoogleQuickSearchBox.apk1
Step 6: Reboot your device, then install the required app which can be downloaded from the XDA thread.
Step 7: Finally, edit the build.prop again with a text editor and ensure ro.build.version.sdk = 15 before rebooting once more.