MikTeX 2.7 is no longer supported and you should install MikTeX 2.8 or later in order to install new packages through the MikTeX Package Manager (which is the program that lets you install automatically).
If you do not want to download all your installed packages again (for example, if you have a somewhat full installation), there is a tutorial to import your installed packages here. You just need to replace
mpm --admin --verbose --repository="C:\Program Files\MiKTeX 2.8" --import-all
mpm --admin --verbose --repository="C:\Program Files\MiKTeX 2.7" --import-all
or wherever you have MikTeX installed.
If you don't want to do this or don't know how, you can just uninstall MikTeX 2.7 and install MikTeX 2.9. However, this may take a long time.