Hi,
Adding the x264 Component to your system does not add an obvious reference to x264 as an encoding choice. What you need to do is ...
Create a new Setting. In the drop down dialog - go Format/Quicktime Movie.
Next make sure that you have the Inspector in Compressor exposed (Window/Show Inspector).
Select your new Setting - and press the Video button up top in the Inspector. Just about half-way down, you will see this:
Press the Change button. The Quicktime Settings dialog appears.
Now, up top in the popup menu - "Compression Type" , scroll down to x264Encoder. After doing so you will be able to define basic settings. You can then press the Options button to open the x264Encoder's Advanced Options.
That's about it. Now you have a custom setting that uses the x264Encoder. You can continue to duplicate this Setting and then edit/rename it to create additional customized settings.
-paul.