Start up Eikon and wait for the automatic script to log you into the database. See the instructions here.
Go to Windows Taskbar > Start > All Programs > "Thomson Reuters" folder > Thomson Reuters Eikon Microsoft Excel
Wait for Excel to start.
In Excel, go to the Thomson Reuters tab > Options
From the Options pop up, go to Apps section and Enable Datastream:
Wait for the Add-In to be installed.
When “Restart Required” appears on the button, restart MS Excel (not the computer):
After restarting Excel, you should see the "Thomson Reuters Datastream" tab appear: