Install path is /var/lib/roundcube for Debian. Don't try to guess it.
---- a/program/include/iniset.php~ 2008-06-09 22:57:53.000000000 +0200
-+++ a/program/include/iniset.php 2008-06-22 12:10:55.000000000 +0200
+Index: b/program/include/iniset.php
+===================================================================
+--- a/program/include/iniset.php 2010-07-17 17:33:04.000000000 +0200
++++ b/program/include/iniset.php 2010-07-17 17:33:23.000000000 +0200
@@ -27,7 +27,7 @@
define('JS_OBJECT_NAME', 'rcmail');
+ define('INSTALL_PATH', '/var/lib/roundcube/');
}
- // make sure path_separator is defined
+ define('RCMAIL_CONFIG_DIR', INSTALL_PATH . 'config');