Hello,

I am using carambola2, and in filesystem i want to add a application.

In rcS i will call that application, Can you tell me how should i add that application in filesystem and build so that i can use it on carambola2.

I tried to put a file in "/build_dir/target-mips_r2_uClibc-0.9.33.2/root-ar71xx/www" and after make, file was deleted from the place and also was not on the board when i flashed it, I am using JFFS2 option.

You can't do that way !
Better see:

What kind of application? It depends if you are compiling some code of your own app, or just adding app from already available apps in Carambola and OpenWrt software repository.

Easiest way is to use OpenWrt Imagebuilder, follow instructions from OpenWrt wiki:
http://wiki.openwrt.org/doc/howto/obtai ... e.generate