After the installation, you may recover your licence key:
- In Steam, right click on your game, the option 'show CD key' (or similar) should appear in the menu. The key should appear as foreseen.
Then, uninstall Steam version (don't forget to reject your licence before),
Download and install DCS-World + the module you purchased on steam (A-10c, Black Shark, CA or P-51d),
Install DCS-W + module.
At launch, use your Steam CD-Key to activate
Fly!
EDIT: Damn Viper you sniped me ;)
EDIT2: I just went to steam. There is THIS message:
"Notice: We have temporarily run out of activation keys for DCS A-10 Warthog. Keys will be automatically added to your Steam account as soon as we receive more."