One of the most persistent “misunderstandings” I see as a technician is when applications are installed improperly (Skype and Firefox, I’m looking at you!). The Mac doesn’t include a guide for how to install software from third parties, and as such, a lot of users are left fending for themselves.
First, let’s take a look at what a Mac application looks like. Generally, it shows as a single file, with icon and name. Most of these are in the Applications folder. This single file is in fact a package with most, if not all, of the the resources that program needs. Unlike in Windows, where one needs to dig through a ton of related files just to find the application file, with Mac applications, you need only double-click on this package to open the software.
When we download a Mac application, it’s often stored within something called a “Disk Image”, in which it cannot be modified. This is to keep the original software intact and unmodifiable by others.
Open this disk image, and it will “mount” on the computer, much like any USB drive or DVD disc would. It can be opened from a new Finder window. Now the downloaded application is sitting in the mounted and open disk image. But it’s not time to use it yet, this is the key step. It needs to be copied to the Applications folder on the Mac. Just dragging and dropping it is enough. Once this is done, we “eject” the disk image just as we would any other disk. Now the new software may be opened from the Applications folder.
The important distinction is that the Application shouldn’t be opened from the disk image, as it may need to make changes to itself or store temporary files within itself, and it’s being opened from a disk that cannot be modified. Opening a copy from the Applications folder is highly preferred.