Note to SMF 2.0 users...
If this isn't showing up in your admin center, you have a couple of options:
Download it from the Upgrades Site
and install it through the package manager
Run the "Fetch Simple Machines Files" scheduled task as follows:
Admin -> Maintenance -> Scheduled Tasks
Check the box in the "Run Now" column next to "Fetch Simple Machines Files"
Click the "Run Now" button