I can't interact with the Steam Overlay. What's going on?

Solution 1:

The Witcher is simply put a horrible "port" that doesn't deserve the name "port".

You could try upgrading the version of Wine it runs on: http://www.insidemacgames.com/forum/index.php?showtopic=43407#entry409385

Now to get technical: The Steam Overlay for Mac is simply a slightly modified and recompiled version of the Windows Overlay. The Overlay tries to detect whether to use a DirectX hook or an OpenGL hook (since both coexist on Windows). The Overlay in this case chooses DirectX, will properly inject but then rely on Windows APIs which are obviously not available, leading to a malfunctioning overlay.

Since you can't tell the overlay what API to hook (Valves detection algorithm does that), this is an issue without a solution you can apply. Whoever ported the game, should take responsibility for this mess and clean it up.

Solution 2:

In the Steam Library, right-click on the game in question and open up it's Properties window. Under the "General" tab, ensure that the "Enable Steam Community In-Game" box is checked. It should be checked by default; however, there are certain situations where it's better to have it disabled - in some games you can eke a few more frames-per-second out of your GPU by turning it off.