Welcome to Railworks and Railworksamerica.
I hope this step by step discription will help you.
1. Railworks Main Menu: select editor
2. In the editor section, select scenario editor on the right side
3. Choose a route, click new scenario, follow the instructions (create a freeroam for the beginning)
4. Once you've created the new sceario, the route should load and you should be in scenario editor mode
5. On the left side of your screen there should appear flyout panels if you hover your mouse over there, you can pin them with the little pin symbol in their upper right side
6. Now select the loco symbol in the second flyout and select a loco from the list
7. Place this loco on a track
9. In the first flyout there you can see a I think partly blue symbol which says Driver, select this driver symbol and now click on your previously placed loco; above your loco should now be a white round symbol with sth. blue in it
As this is a freeroam there is nothing more to do. You can add wagons with the same method, but from the wagon list and you don't need the driver icon for them.
10. Click on the orange "play" symbol in the down right side. Now your scenario should load and you can click on the engine and explore the route.