If I'm reading this correctly you're trying to load MYApplet.jar from the model/lib/userlib directory.
This directory isn't served via the HTTP interface, so there's no path for it. What you probably want to do is bundle the MYApplet.jar archive in a theme package so it can be served up as an asset.
You can't acccess ../model trough an url (that would make it possible for anyone to just download your model from the server).
If you want to access an applet public, you should put it somewhere in the web folder.