Cycles is indeed an add-on, as your error message suggests. I don't remember if it's enabled by default or not, but nevertheless it seems to have been disabled.
To enable it:
Open up user preferences (File -> User Preferences or CtrlAltU).
Go to the Add-ons tab.
Click Render in the Categories list to the left.
Make sure the checkbox next to Render: Cycles Render Engine is checked.
Click the Save User Settings button.