You may download content from several sources, the DLS and third party websites are most common. The DLS is accessed either through the website (the Black Pages) or the Content Manager. Auran is happier when you use the Content Manager, so let us look at that first.
Start Trainz by clicking on the icon, when the small start up window opens choose the "Content" tab. This opens the Content Manager. Allow it to connect to the DLS for a couple moments and verify that your user name and password have been accepted. If there are no issues at this point, then proceed to the tabs in the manager window. You can start by viewing the content on the DLS by choosing the "Download Station" tab. At that point you can use the filters in the search option on the left of the window to further narrow down your parameters.
Once you find the item you are looking for, you can right click on the items listing in the right side window and choose "download".
After downloading through the Content Manager, the content will be installed for you. At this point, it may be flagged with warnings or errors. Warnings can generally be disregarded, but errors cannot.