SP Flash tool mediatek - cannot add download agent
I finally discovered that the problem was in the path to download agent, where was the Chinese characters for example : 

**x-302 NEW firmware \ 线 刷 工具 和 驱动 \ mt65xx preloader 驱动 **

I just retype to :

**x-302 NEW firmware\Flash\mt65xx preloader**

and then it works correctly and retrieve the download agent. In all the tutorials I could see that the used path with Chinese characteristics and it worked. However, in my case not.

Here is some helpfully link : 
Tutorial flash device with mediatek chip 1

Tutorial flash device with mediatek chip 2