Ideas:
- Stick to PDE (just joking!)
- Look around for some other Java tutorials on Eclipse?

What do you mean by " I can't add the core.jar". Add it where? What did you try?

[EDIT] Mmm, perhaps it is in the first step of the tutorial? The problem with Eclipse is that it evolves quite quickly, so some tutorials can be obsoleted.
The way I would do (from memory, details can vary), once you made your project, is to go to Project > Properties, then to the Java Build section, Import libraries, add external jars.

Sorry phi.lho
My bad, I couldn't pick it up in step 3 of the tutorial (fuzzy) ...but the thing was I had to select the folder first (shame on me)

Thanks