To Install MP Stacks + SocialGrid, download the plugin from your account above, then go to "Plugins" > Add New" > "Upload" and upload the zip file.
Alternatively, you can copy your license key from your account above and then in your WordPress dashboard go to "MP Stacks" > "Add Ons", paste in your license key and click "Install".
To authenticate Twitter, Instagram, and YouTube, in your WordPress dashboard go to "MP Stacks" > SocialGrid Settings" > and follow the steps carefully on that page.
If you have followed the authentication steps but it is still not working, it's possible that you just missed one of the final steps, which requires you to click on a link in order to activate the authentication. This is just for Instagram (Step 10) and Twitter (Step 11).
If you find your post are squished off to one side and they are very small, try switching the Brick "Alignment" from "Left/Right" to "Centered".