Unmarshalling Error while registering an object in RMIRegistery ClassNotFound

Hy first of all i know it's a duplicated post i have seen the "next guy's here" post (and about 30 of the same kind searching on google)

I run eclipse 1.3.2 Helios on a 1.6 VM

I do have a running example, works like a charm (no additional set-ups or configurations made , just new project/java project etc).
But when i try to do the EXACT same thing in another project it throws this exception (can not find the interface of the remote object).
(And by "exact" i mean even copy/paste the classes from the old one to new)

I read about configuring the class-path and i wander
1) Class-path means include this when you run the application (or am i wrong?)
2) Since the interface , the remote object and the class that creates it are in the same package(witch means same location) why is necessary to specify a class-path
3) And how to change that class-path ?
4) Does the VM argument specify where from to download the class?
throw Run/Run Configurations it tells me "default class-path" (the same as the working example)

If anyone can explain (briefly) these concepts or to point where im wrong i guess that will shed some light on my problem

Thanks in advance
And don't flame (don't flame hard i mean :

)