Before opening, move the DMG to your Applications folder or a dedicated SnowLeopard folder. Right-click the DMG and select (avoid double-clicking; this mounts it automatically but can trigger Gatekeeper on newer Macs).
You may need to disable Gatekeeper temporarily via Terminal: sudo spctl --master-disable (Re-enable with sudo spctl --master-enable after installation.) download snow leopard 1068 dmg google drive upd