for Mac was never distributed as an installer but only as a disk image (dmg file). If you have opened the image you see it as a device (disk) in the left side list of the Finder.
If you have not closed already you will see also a window with the content of the disk image. To install OpenCPN drag the icon OpenCPN in this window into the application folder.
To close (detach) the disk image click onto the arrow right of the device item. Because it was not installed yet you cannot uninstall it but only unmount the disk image.
Hopefully some day there will be a real installer for OpenCPM Mac.