Installing the addon
Open Blender and go to Edit > Preferences
Navigate to the add-ons tab in the sidebar
Click the install button, and select the file named "alpha_trees_2_0-INSTALL"
Then click the checkbox next to the addon name, and it's installed!
Installing the library
Find the file named "alpha_trees_lib_2_0-EXTRACT", and extract it to the directory where you want the library to be.
Copy the path to the folder you just extracted into the red highlighted section called "library path"
You can now use all the trees in the chosen library from the 3D viewport
If you don't want to use a premade library, you can also select any other folder, and it will have the required directories automatically created.
If you want to use a library that you created in a previous version (before v2.0), you can select it as normal, and it will automatically be updated to the new standard. Be careful, though, as once you do this, the library cannot be used in older versions any more.