Download the installer directly to your Mac. Note: You will still need a valid serial number to unlock the software from "Demo" mode to the full version. Step-by-Step Installation Guide on Mac
Native Instruments provides legacy installers for users with existing licenses: download traktor pro 2 mac
This is a quarantine flag issue. Open Terminal (Applications > Utilities) and type: sudo xattr -r -d com.apple.quarantine /Applications/Traktor\ Pro\ 2.app Press Enter, then launch the app. Download the installer directly to your Mac