I had that same error and for me it occurred because I didn't specify the Private Library Path. After I set the path to any old folder and deleted the auto-created Arduino_Uno project (I guess it's called the core project?) it worked for me.
I have a different problem that relates the serial monitor. I'm working with the current version of Eclipse CDT and 1.2.2 Arduino plugin on OS X Lion. For me there is no Arduino Folder under Window > Show View > Other.