how to import new master page in kentico

What development mode are you using? Portal OR ASPX? If ASPX then you need to create an instance of that page template within Kentico and ensure it's in the website's file system. If Portal, you don't do any type in import, you have to implement what you have in you master page into Kentico's page templates and utilize layouts, zones and webparts.

Kentico strongly recommends using Portal mode over ASPX because it's simple, extensible, flexibile and easily upgraded. Whereas with ASPX, most simple changes to a page or template would require knowledge of asp.net.

I use ASPX mode and now I design the master page in Visual studio, but I do not know how to define the master page in kentico. after this action, I will create an instance of the page template base on master page. please guide me. thanks

I'd strongly recommend rethinking your design and NOT using ASPX mode. ASPX mode is simply for legacy apps and essentially you will lose a lot of functionality you chose Kentico for.

Here are some docs on how to get started and developing with Portal mode.