If you want to install it on a recently downloaded Netbeans (7.1 in my case), first go to "Tools"> "Plugins", select the "Updates" tab and install the updates. Then, after rebooting, you will go to the plugins mentioned above, and now you can install all the necessary updates (among them there will be an HTML editor).
Simply put, as mentioned in @skripted, just install the plugin by following the instructions on this page .
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