In order to have BrandMyMail buttons on your Gmail, you need to install BrandMyMail extension for every browser/pc that you use.

- Go to Settings and click on browser on section "Connect BrandMyMail to your Email Client"
or click directly on the following link 
- Click to install extension
- After installing extension, reload Gmail (in Firefox you need to restart the browser)

You can install BrandMyMail as well for other clients.