In order to change the JRE to be used by KNIME go to Eclipse Preferences and select the Java menu.
Afterwards you can add the right JRE. Under MacOs you choose the entry MacOS X VM
then press next and select the right path, which should be /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home as shown here:
Press Finish and the right JRE will be added.
Afterwards you have to set the compiler options. In order to do so go to Project and select Properties.
No select Java Compiler and select the correct JRE at the Compiler compliance level:
If you run the project now KNIME should start without problems.