The address of Szechenyi Bath and Pool, Budapest is
9-11 Allatkerti korut Street, Budapest, District 14 (on maps, you will see District XIV), H-1146. (i.e. the post code is 1146)
Budapest, XIV. kerület Állatkerti körút 9-11. H-1146
If you want to send a mail by post, please use the official Hungarian name of Szechenyi Baths and Pool in Hungarian, which is
Széchenyi Gyógyfürdő és Uszoda.
See the location of Szechenyi Spa Baths compared to the river Danube, and downtown Budapest (in the left hand corner in the bottom):