Yes, it is free. If you go to the App Store, sign in, and select Yosemite, nothing happens - is that correct? If so, try going back to the Featured tab and on the right side under Top Free, hover your cursor over Yosemite. A download button should appear. Click that and see if it works. If that doesn't work, try resting the App Store by following the reset link below.
Reset Learned from Old Toad
Check that your computer is compatible with Mountain Lion/Mavericks/Yosemite.
To check the model number hold down the option/alt key, go to the Apple menu and select System Information.
MacBook Pro (Mid/Late 2007 or newer) model number 3,1 or higher
Your Mac needs:
- OS X v10.6.8 or OS X Lion already installed
- 2 GB or more of memory (More is better - 4 GB minimum seems to be the consensus)
- 8 GB or more of available space
Check to make sure your applications are compatible. PowerPC applications are no longer supported after 10.6.
Application Compatibility
Applications Compatibility (2)
Do a backup before installing.