OpenShot Encode video fails, but libraries installed

I am a new OpenShot user on an eeePC 1000 running Ubuntu:
Linux 2.6.32-38-generic #83-Ubuntu SMP Wed Jan 4 11:13:04 UTC 2012 i686 GNU/Linux

First test to try it the software: I import some video clips to a new project, lay them on the timeline, and attempt to export the video.

I select web, and various options, but by the time I hit the last one, "quality", I get an error message:

[BEGIN ERROR]
The following formats/codecs are missing from your system:

libx264
libmp3lame

You will not be able to use the selected export profile. You will need to install the missing formats/codecs or choose a different export profile.
[END ERROR]

However, these libraries appear to be installed, at least to the best knowledge of the provided Synaptic package manager (GUI). "apt-get -install" didn't find these packages by name:
  507 sudo apt-get install libx264
  508 sudo apt-get install libmp3lame
E: Couldn't find package libx264
E: Couldn't find package libmp3lame

In Synaptic I have already installed:

1. libx264 [it won't let me paste the technical description but in properties it says "2:0.85.1448+git1a6d32-4" and that it's the installed and latest available version:

x264 video coding library

libx264 is an advanced encoding library for creating H.264 (MPEG-4 AVC)
video streams.

This package contains the libx264 shared library.

2. libmp3lame: actually it's called libmp3lame0 here.
Again, latest available version is the installed: 3.98.2+debian-0ubuntu3

Thanks for any help understanding why!

---
Moreover, I can't get rid of the export window (cancel doesn't work). -- Oh, whups, the error window had come up again and was hidden. (A different problem.)

This happens on a freshly rebooted system with only the OpenShot application open.