You'll see the button "Download" next to the Yosemite entry.
Click that Download button, and you will get the dialog box with a Continue (or Cancel) button.
If you get nothing like that - then boot to your Restore partition (Restart, holding Command-R), and choose to reinstall OS X. That SHOULD download the current Yosemite installer.
(Tyler23 answer is better than mine, but I think should be a option-click on the button)