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Open Tools -> Plugins - > Downloaded. Add new Plugin which is not signed and has License Agreement. (plugin can be used from attachment). Then press the following: ALT+I(install) -> ALT+N (next) -> ALT+A (accept) -> ALT+I(install) -> ESC (exit from wizard). After this combination the plugin is checked as chosen for install, but it should be unchecked. Try to uncheck it. It possible to do only in second time. In the first - no reaction. The reason is that after cancel of installation plugin is unchecked but the panel is not repainted. This issues is found on JavaME SDK3.0, but Plugins UI is provided by NetBeans platform.
Is it reproducible in the latest builds of NetBeans IDE? http://bits.netbeans.org/netbeans/trunk/nightly/latest/
Should be fixed in http://hg.netbeans.org/core-main/rev/fe881edfd9c1