Hello sandeepsandeepky@gmail.com,
You can just rename the file from <toolbox>.zip to <toolbox>.mltbx
You can right click the file -> properties and in the general tab you find the name (in the example below the name is lpuart_s32k_transmit and the extension is .tlc; just rename the extension).

Hope this clears things up a bit.
Kind regards,
Razvan.