MySql master and slave design I have set up mysql replication with single slave.
Whenever i am adding data to master database, it is automatically copied to slave database.
But if I add data to slave database it is  not available in master database. why?

If it is wrong to add slave database, how I can tell to the slave database to redirect writes to master? is it possible?

My application is designed as normal application not for replicated database. Should I  change my application code for working with replicated database?
Data should never be directly inserted into a MySQL slave [except for edge cases ignored here].  To prevent this accidentally happening you should set 
    read_only
in your my.ini.  Details on this option are here: http://dev.mysql.com/doc/refman/5.0/en/server-system-variables.html#sysvar_read_only

MySQL Proxy might be an option for read/write splitting, but the last time I checked there were bugs that may mean this is currently unworkable. (MySQL Proxy: http://dev.mysql.com/downloads/mysql-proxy/, R/W split: https://wikis.oracle.com/display/mysql/MySQL+Proxy+RW+Splitting)

A third option (if you are looking for more HA than performance, and wish to avoid rewriting your application) would be to configure Master Master replication and put a loadbalancer such as haproxy in front of it - there are some excellent tutorials on the web on configuring haproxy in this kind of environment. (HAProxy: http://haproxy.1wt.eu)

(edited for spelling/links)