Installation problem

Hi all,
I using ubuntu 14.04
on installation step:
- Run apt-get install nginx-plus to install nginx-plus package
I have such error: - E: Unable to locate package nginx-plus

and on step:
- In order to upgrade from the previous version of nginx-plus, run apt-get update and apt-get install nginx-plus
I have such errors: -
Err https://plus-pkgs.nginx.com trusty/nginx-plus amd64 Packages
error reading X.509 key or certificate file
Err https://plus-pkgs.nginx.com trusty/nginx-plus i386 Packages
error reading X.509 key or certificate file

could some one help me?