Fuel pulls all your pending orders from Shopify every 20 minutes. If you are not seeing your order right away, give it up to 20 minutes for it to show up on your Orders page.
If you are still not seeing your order after 20 minutes, it was most likely not created in the correct way for Fuel to be able to receive it.
Most common reasons your orders are not showing up:
All products you want to be fulfilled must be created through the Fuel app
Order must not be archived
Order must be marked as "Paid"
If your orders did not meet these conditions at first, you will want to create a new manual order with these conditions for us to be able to fulfill your products.