So plug in a USB keyboard ( even a wireless one with a USB adapter should work ) ...

Do you have a Bluetooth device/adapter in your pc ???

I installed Lion on my laptop and it said the same thing so I just plugged in a USB keyboard I was using for my desktop and the message disappeared.

Now the trick is to go into the Bluetooth settings in the System Preferences and click on Advanced and uncheck those 2 items for mouse and keyboard so as to prevent the message from reappearing at every logon ...