Pin For Pintrest

The pin it button on my browser ( Firefox and Safari) works really great but the app will not down load onto my macbook air. I have it on IPad and it is great, but I would like to use it on my macbook air too as I often lend my grandson the Ipad. I have tried installing it from the app store and the Pintrest site it just will not appear even thiugh I am told it is installed.

GrandmaB1 about Pin for Pinterest, v1.7