You may need to make sure that the case is correct for the "User Library" you created.
I recommend that everyone use all lower case opencv.
Also, the User Libraries are specific to your Eclipse "workspace".
Check if your opencv User Library exists (under Window->Preferences->Java->Build Path->User Libraries).
Also, try right-clicking on your project, and doing Build Path->Configure Build Path, to see what libraries it is looking for that it can't find?