Within the eclipse config file (%ECLIPSE_HOME%configurationconfig.ini) make sure the following line exists.
osgi.framework.extensions=net.rim.eide.preprocessing.hook
With the current BlackBerry plugin (1.0.0.67) config line is added for you. I'm not sure about older versions of the plugin.
Also, checkout this Stack Overflow question for more information on the BlackBerry preprocessor.
Preprocessor directives supported by the RIM compiler
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