Should you need to locate a marketplace order, this is located in the Marketplace Center section of the Retail tab.
Clicking on this brings you to a search screen where you can enter criteria for the order your looking for. If you are just looking at current orders in your store then simply press "search" without changing any search criteria and you'll be given the orders waiting to be sent from your store.
If you are searching for a specific order you can use any of the criteria seen on your screen.
Once you enter your search criteria you can click "Search" and you'll be presented with orders that match.
Note: the screenshot above is not representative of every user's marketplace orders screen. you may have less information/buttons available to you depending on your position.
You can then order these as you would items during an item search by clicking on the column names.
For each order you can view all sorts of information at the bottom of the screen. If you've done a Pick List you may recognize this as the screen you use to mark an item as "No Stock" (in the screenshot above, the "Transfer" button has taken the place of "No Stock," but will appear as "No Stock" for most users).