Metamask: Add Arbitrum Testnet Network

To configure Metamask for the Arbitrum Test Network we must add a new RPC Network. For this, open Metamask, access the accounts menu at the top-right corner and select Settings:
Scroll down to the Networks section and select it:
At the bottom click on Add Network and fill in the following data:
Finally select the Arbitrum Rinkeby in Metamask's network selector (drop down at the top of Metamask):
You are connected to the Arbitrum Rinkeby now.