[java-dev] could not load class ...

i am new to max/msp/jitter and i am trying to create a mxj external but it doesent really works.. i use max/msp version 4.6.3 jitter 1.6.3 and java 1.6.0_02-b06.. max window says Jitter java support installed.. no error message nothing.. but if i try to write a java class there it writes error: Could not load class xyz... can anybody give me a hint what could be wrong.. i've searched the forums but i did not find anything about that..