The activation code is now tied to your Steam account, so it will not work. What will likely happen is that the game will check your account, and then start downloading the complete game again (which is quite large if you have any of the DLC... last time I played it was taking up about 50GB).
To re-install from the disc, follow the instructions found here on the Steam support site, though be aware that there'll be several patches that need to be downloaded (including possible the transition from TS2013 to TS2014).
A more productive use of your time, and bandwidth, would be to try and figure out what is going wrong when loading the scenarios. Do you get a specific error message?