Then ask the xcache developers on how to enable xcache successfully in apache. All successful reports that I found are related to lighty, as xcache belongs to the lighttpd project and php fcgi on apcahe and lighty works a bit differntly (apache does not reuse connections if I remember correctly). So better try eaccellerator or ask the xcache devs on how to configure apache and php-fcgi so that xcache works with apache too.
As I told you already above, this problem is not a ispconfig problem. It wold be the same as if you ask how to enable xcache on a Inter processor because your server has a intel processor. ISPConfig does not interact with apache - php-fcgi and xcache, ispconfig just writes a standard apache config file.