With a big forklift truck

(sorry I could not resist the opportunity).
Seriously, two things need to be configured first.
- the open wagons must be industry enabled and set to load 20 and 40 foot containers. Not all rolling stock objects are industry enabled.
- the loading/unloading point must be an industry or industry track also set to load/unload 20 and 40 ft containers.
The first step is to test your open wagons to see if they will carry the containers. This is easier in later versions of Trainz (e.g. TRS22/Trainz Plus) and you did not indicate which version of Trainz you are using. But in general in Surveyor 2.0 (TRS22/Trainz Plus) click on a wagon, open its context menu, select the option
Edit Details and see if a section labelled
Commodity Levels is present. If not then the wagon is not able to be loaded with anything. If it is present then further instructions can be found on the Trainz Wiki at
How_to_Use_S20_Palettes#Managing_Consists.
The second step is to select an industry or industry track (basically an industry with track only and no buildings so you can add your own) and perform the same test. More details can be found at
How_to_Use_S20_Palettes#Managing_Industries.
It is a learning curve, sometimes frustrating but always interesting.