Set up Max’s classpath to find your Java externals. Go to the file [Max installation]/Cycling ’74/java/max.java.config.txt. Make a backup! Edit the file and find the line:
; max.dynamic.class.dir /Users/topher/myclasses ;

Make a copy of this line and remove the leading semicolon and change the path to the bin folder of your Eclipse project, so it should look something like this:
max.dynamic.class.dir /Users/adam/Documents/workspace/max/bin