This error "Android Compatibility JAR not found:" popped up at me this morning.  I had

1. Right clicked on my project
2. selected "Android tools"
3. and added the  "Add compatability library"
4. Mega output but at the end I got this [2011-12-08 12:07:27 - Android Compatibility JAR not found:] /home/ayo/v4/android-support-v4.jar

After a couple of hours fiddling about it sans success, it seemed the only way to save my self any further hear pulling was to manually add it to the project.

So I,

• Right clicked on the project
• selected the Java Build Path
• chose the Libraries tab
• selected the "Add External JARs.."
• and navigated to the folder wherer the errant JAR was hiding
• /home/ayo/ToolsNUtils/DROID/android-sdk-linux_x86/extras/android/support/v4/android-support-v4.jar

After this all was well.

Bon Chance