luchozero, seguramente ya seguiste todas las instrucciones para instalarlo,
1. Upload to your plugins folder, usually wp-content/plugins/ and unzip the file, it will create a wp-content/plugins/wp-cache/ directory.
2. If you have Gzip Compression enabled turn it off (in Options->Reading).
3. Create wp-content/cachedirectory and make sure the web server can write in it.
4. Make wp-content writeable by the web server.
5. Create a symbolic link from wp-content/advanced-cache.php to wp-content/plugins/wp-cache/wp-cache-phase1.php. ln -s wp-content/plugins/wp-cache/wp-cache-phase1.php wp-content/advanced-cache.php
6. Add the following line to your wp-config.php file: define(’WP_CACHE’, true);
7. Go to “Options” administration menu, select “WP-Cache” from the submenu and enable cache.
Acá más detalles:
http://mnm.uib.es/gallir/wp-cache-2/