Start LFS and enter a race (offline mode is recommended for testing). Minimize the game and open the Tweaker. "Attach to Process" tab, select the running LFS instance and click "Attach".
If you love LFS and race online, buy the S2 license. If you need the tweaker purely for the visual debug tools (tire heat, FFB graph), consider it a utility. Do not use it to pirate online. lfs pro tweaker 06r fixed