Orit, the Membase UI presents this information as the graph labeled "Cache miss ratio".
To determine it yourself, you will need to calculate it with ep_bg_fetched (how many items have been fetched from disk) versus get_hits (how many items were serviced by the system overall).
-'hit_ratio' is not something the server provides directly, it is usually computed by an external system by measuring get_hits versus cmd_gets but this refers to the hit_ratio of the cluster (key not found) as opposed to coming off of disk.
-'get_hits' are the same as memcached and refer to how many gets were serviced with a key that the system had
-'get_misses' are also the same as memcached and refer to how many gets were serviced with a "key not found".