To add a meta description to your store, go to System->Setting->Option, where you can add the 'Meta Tag Description' at the bottom of the page.

If you want to add meta keywords to your store, you can go to http://apps.miiduu.com/ to install the app ' Custom Meta Tag' first and then go to your administration->App Store->Installed Apps->configure the 'Custom Meta Tag' app to add the 'Keywords'.