I'm not sure whether XAMPP actually has the sites-available/sites-enabled, mods-available/mods-enabled structure that the Ubuntu Apache packages use. But if those directories are there, I would expect them to be somewhere in /opt/lampp/etc, because the main config file httpd.conf is there (according to http://www.apachefriends.org/en/xampp-linux.html#385).
According to some (quite dated) blog entries on XAMPP, there might be a fitting LoadModule directive in that httpd.conf, something to the effect of
#LoadModule rewrite_module path/to/mod_rewrite.so
If this is still current, you need to remove the comment symbol (#) from that line. Additionally, you need to enable local overriding of the central settings (for example from a htaccess file or a vhost). So you need to look for
and change it to
That may appear in the httpd.conf more than once, you need to adjust every occurence. After the next restart of Apache mod_rewrite should be working.
At the end of the day, the mod-available/mod-enabled system is "just" a conventient way to manage configuration snippets like the LoadModule above, see for example /etc/apache2/mods-available/rewrite.load from the Ubuntu Apache packages).