Your problem is that the Eclipse project is already configured to work on version 6 of the system libraries. You however do not seem to have this specific version, therefore you need to reconfigure the build path for the project.
In order to change your JRE System Library, you must
- Go to
Project|Properties
- Choose the
Java Build Path
section
- Select the current JRE System Library, and choose
Remove
- Select
Add Library
, and under the JRE System Library
category pick your system library; you probably just need to choose the Workspace default JRE
If the default JRE version is <= 4, you most likely will have problems building the project. In that case I suggest you install a more recent JRE.
More generally, under Window|Preferences
, you can check Java|Installed JREs|Execution Environments
to know which of the installed JREs match a specific execution environment. Should be pretty clear, but that page demystifies things a bit.