On your startup splash panel click on "Content Store" and download from there. I don't have Platinum (I have Gold/Plus) but I think the Content store works the same in both. Items you are entitled to should say "Download" while items available for purchase should have a price. Also note at top you can click on "Purchased" to filter.
Once installed start the game, select Driver Surveyor, and your routes should appear at bottom. Select a Route and click "View Sessions" to see any sessions included. Sometimes you need to create your own session (if none came with game) and you need to "Create Session" - Click that then route/session will load, place your desired loco/consist etc (bottom "handbrake" icon on right) and then select Driver to drive.
. Today i buy and i install trz2019 platinium edition and i have no platinium content into... I have only one road and its kickstarter county 2. How i can play my platinium content???