Go to the Plugins Manager and verify that the modules that your project is using is marked as "Active". You may find that the needed modules are currently "Inactive".
In my case, the error message started appearing after an attempt to update my NetBeans 6.1 application. During the process, one of the modules failed to install properly. A warning appeared stating that additional modules failed to install due to the initial problem. After multiple cycles of updating NetBeans 6.1, the error message about the missing plug-ins persisted. After reviewing the "Installed" tab on the Plugins Manager and activating the needed modules, the error message went away.