I thought you'd never ask . . . Just follow these steps in the scenario editor:
Step 1: Open the
Browser Tab by sliding your mouse toward the middle-left portion of your monitor; once it's open, click on the pushpin icon in its upper-right corner to hold it open. Once it's pinned in place, click on the small, blue cube with a yellow arrowhead on it (just below the larger , blue cube in the Tab's upper left corner).

Step 2: Clicking that small cube opens the Assets Options Tab in the upper-right corner of your display; click on its pushpin to hold it open. This Tab contains links to all of your TS assets.

Step 3: Click on the downward-pointing arrow to the right of the top name in the list to open the assets list. Note that assets are listed by maker's name!

Step 4: Scroll through the list until you see the name you want, then click on it to open that asset maker's properties box. This list contains the name of each asset and asset selection boxes.

Step 5: Click in the middle box alongside the asset you want to load. Its contents will immediately appear in the Browser Tab, from which you can now select and place them.

If you make a mistake, follow these steps again to unload the assets you mistakenly loaded by unchecking their box (click the yellow arrow in the box) and then selecting the desired assets.
Keep in mind that this process loads assets only for the scenario you are currently editing, not for the route as a whole. Your choices will be passed on to clones of the current scenario, but not to any new scenario.