I just installed NetBeans IDE 6.0. I also have SUN Java Studio, and NetBeans 5.5.1 already installed. They also seem to have a Derby database for them. When in NetBeans 6.0 I try to connect to any of the sample databases or even one that I create, I get message similar to "....unable to connect. cannot establish connection to using org.apache.derby.jdbc.ClientDriver (Unable to find a suitable driver).
To fix the issue I simply expand the NetBeans IDE | Services tab | Databases | Drivers | Java DB (4 of them) and add the path to the correct Derby database driver location. I just right clicked each of the drivers and chose Customize menu item. Then added the following path (customize it to your system) C:\Documents and Settings\MyProfileHere\.netbeans\6.0\jdbc-drivers\derbyclient.jar.