I have the same platform (netbeans 8.0 and JDK 8), searching in other project (build in netbeans 7.4 and JDK 7) I found a little difference in the project.properties file, search it in <>/nbproject, look for the line:
javac.processorpath=\
${javac.classpath}:\
${libs.eclipselinkmodelgen.classpath}
and change the text to finally be the following:
javac.processorpath=\
${javac.classpath}
thats all, I don't know if it's a bug in netbeans 8 or maybe it's something wrong in the way to make the project, but this fix work for me, more details in this blog entry (spanish version)