Note: You must install OpenCPN into your Applications folder where it belongs. The installers of the plugins of course expect it to be there.
OpenCPN is distributed as a Mac ".dmg" file
Download the .dmg file.
Open the dmg file by double-clicking on it - this will mount the dmg as a virtual device and create a mount on your desktop.
Drag the OpenCPN.app icon to the Applications icon in the installer window. This may require the administrator password.
Unmount the dmg by right-clicking on the desktop icon and selecting “Eject”
You can have a look at this tutorial How to Install and Remove Apps for general instructions on installing Mac OS X applications.
If you have a message that installations are only accepted for programs from qualified developers, navigate to Applications in Finder, Control-click OpenCPN.app, select Open from the context menu and confirm that you want to start it in the displayed dialog.
To save a previously installed version rename the app in the Applications directory before installing the new version.
If OpenCPN.app is replaced, the installed plugins will not be there anymore and have to be re-installed on the new OpenCPN.app. The userdata of the plugins will still be there.
Plugin Installation
Download the installation package (*.pkg)
Make sure that the OpenCPN application is not renamed. The plugin will only install on OpenCPN.app
Double-click the downloaded file to start the installation. In case your security settings are tight, which is the default on modern macOS versions, Control-Click on the file, select Open from the context menu and confirm your intention to open the installer.
Follow the instructions provided by the installer
In case your security settings are tight and the OS refuses to start the installer, control-click it, select Open from the context menu and confirm your intention to install the plugin in the subsequent dialog.