Macs running OS X Yosemite have a native font management tool called Font Book. To add a new font to your Mac, download the desired font, open it in Font Book and click "Install Font" to make the font available to all compatible applications on the computer.
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Download the font you want to install from one of several free font websites, such as DaFont, Font Palace or 1001 Free Fonts. If Mac and Windows versions of the font are offered, download the Mac version.
Click the "Finder" icon on your Mac's dock to launch the Finder.
Click your home user account folder on the menu on the left of the Finder window and select "Downloads" to view your recent downloads.
Double-click the downloaded file to extract its contents and then click the folder containing the extracted files. Some fonts are not contained in folders and will be directly clickable in your Downloads folder after you extract them.
Click the font file to open it in Font Book.
Click the "Install Font" button to add the font to your Mac.