It looks like the whole clustering component JAR is not in the classpath. I remember that I once dealt with a similar issue in Solr 1.4 and the cause was the relative path of the <lib> tag being resolved against the core's instanceDir, which made the path incorrect when directly copying and pasting from the single core configuration. Try correcting the relative <lib> paths or replacing them with absolute ones, it should solve the problem.
Cheers, Staszek
