You should see the FactSet button in the ribbon menu. If you do not:
In Excel, go to Home > Add Ins (if you don't see Add Ins under Home, try the Insert tab).
You need to search for FactSet 365, and if you don't see a search box press More Add-Ins.
On the FactSet 365 result, click Add > Continue.
You will need to Sign In to FactSet using your FactSet account.
If you have a personal license to Microsoft Excel (e.g. when you sign in to Excel you're not using your Stanford credentials), you can follow steps 1-3 above.
This will NOT work if you're using Stanford-licensed Excel, so in that case please use either the Trader's Pit computers of the Virtual Desktop.
Troubleshooting
"Unknown FactSet ID" - If you're not able to log in to your existing Stanford FactSet account, it may have expired. FactSet automatically deactivates accounts if they haven't been used in 90 days. Clicking on the forget password or username links will not work, as you'll never receive the email because they removed it from the system.
Call the support number at the bottom of the FactSet log in page and explain the situation. They should be able to forward the request to the account team to reactivate it.