Migration 0.9.840 --> BETA3, skins problems

Hello, I managed to reinstall a tomcat/mysql, imported the data from my "production" mysql instance into my "testing" mysql instance, and launched tomcat (with updateschema=1, and manually set a default value to XWD_ARCHIVE into mysql schema) targetting this mysql instance. I had a customized skin, as a result when I access Main/WebHome, I get a page with a nearly unreadable skin. I'm trying to apply the piece of advice from http://www.xwiki.org/xwiki/bin/view/Main/ReleaseNotesXWiki10Beta3 to migrate to the new skin, but I have two questions: 1 - I just cannot log in my new xwiki with my old registered users, and I can't neither with the "Admin"/"admin" user. So I cannot update the preferences in any way ... The log in page just refreshes itself indefinitely. 2 - Even if I could log in, how can I install the panel application with the import tool as my own skin won't display the Import tab (it does not display many things anyway ...) ? Thanks for your help, Jérémie Bousquet