The version of XoWiki, you are able to install depends on the version of OpenACS, you have installed. So, in a first step, you have to figure out, what version of OpenACS you have (look into http://YOURHOSTANDDOMAIN/acs-admin/apm , line with acs-kernel)
If you have at least OpenACS 5.3, then browse to http://YOURHOSTANDDOMAIN/acs-admin/install and click on "upgrade from repository", and you get the latest stable version installed.