http://www.openacs.org/xowiki/XoWiki_User_Guide
there is one by solution grove
http://www.solutiongrove.com/docwiki/howtoxowiki
to answer you questions: to get to the file-selector, edit a page and click on the image button in the toolbar. In earlier version, you came straight to the OacsFs, in newer versions of openacs, you might have to add OacsFs to the list of xinha plugins in the parameters of acs-templating (from admin/site-map)
if you are in openacs (no dotlrn), the globally mounted file storage is used. if you mount xowiki from within an openacs community, the folder of the openacs community is used. If this is not sufficient, check the configuration options of http://openacs.org/api-doc/proc-view?proc=template%3A%3Awidget%3A%3Arichtext
to make xowiki a start page, add the following .vuh file
http://alice.wu-wien.ac.at:8000/xowiki-doc/#package-prefix
to the toplevel www directory