Steam – I can’t screenshot on Steam

Ah yes, this is a wonderful message, similar to when Steam sits for hours updating itself. Some things to try:

• As Samjus notes, make sure Steam has access through your firewall. Add an exception for Steam or try disabling it (make sure it doesn't turn on the Windows firewall when disabled). You might also need to check your router to ensure it's not blocking the needed ports.

• Exit the client, then navigate to Steam's folder (probably C:\Program Files\Steam on Windows) and delete the ClientRegistry.blob file, then try again.

• Give up and try again later; sometimes this happens when Valve is having server issues.

• Uninstall and reinstall Steam.

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.