Go to Tools > Library Package Manager > Package Manager Console and type:
Install-Package Owin.Security.Providers
Open your App_Start\Startup.Auth.cs, and add:
using Owin.Security.Providers.Steam;
Then, in the ConfigureAuth
method, add:
app.UseSteamAuthentication("your API key");
You can get an API key from http://steamcommunity.com/dev.
A "Steam" button should be added to your login page.