To correct the icon, simply drag the file to or from the Desktop. The icon should change to an icon of a StyleWriter printer. (In rare instances you may have to rebuild the desktop to correct the icon. To rebuild the desktop, restart the computer while holding down the command and option keys. When the desktop appears, you will be asked if you want to rebuild the desktop. Click OK.)
Once the icon has been corrected (looks like a printer), you can drag it to the System Folder. You should be prompted that the file is an extension and needs to be moved to the extensions folder. Click OK. The file should show up in the Chooser.