Make sure you've deleted any older versions of the DLC and the base game, then restart your PC.
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When you open the game, it should display the current version number in the top right corner (version 5.5.2 is the latest) ^^ However, it doesn't display the version number of the DLC since it's typically meant to mirror the base game. The only way to know if you're using an older version is via the "incompatible version number" warning you received.