Once you've downloaded the Steam version of the game using the key provided, you'll be able to transfer your trial progress by following these steps:
1) Launch the trial version of the game:
2) The following options will appear:
3) Select the option to 'Copy trial progress to Steam™'.
4) The following message will appear in the bottom left-hand corner of the box: 'Saved games transferred.'
5) You will now be able to carry on from where your trial ended by launching the game on Steam.
Please note: If you have Steam running in the background, when you click 'Copy trial progress to Steam™', the Steam window will close, in order to transfer your trial progress.
If you then click "Launch Steam™ version of the game" then Steam will restart and launch the game with the progress transferred.