The best explanation about ICEfaces can be seen at their website, http://www.icefaces.org/main/home/.
Now, the step to install it on the NetBeans 6.5.1:
1. Run NetBeans 6.5.1.
2. Navigate to the Tool -> Plugins. Go to Available Plugins tab, type "icefaces" on the search box. It is already there. However, they are not the latest version. Go with them if you just want to try ICEfaces. Install them and you are finish. Otherwise, follow the next steps to install the latest version (as for today it is ICEfaces 1.8.1 20090526).
3. Using your favourite browser go to http://www.icefaces.org/main/downloads/os-downloads.iface?category=NetBeans look for the plugin for NetBeans 6.5. Downloand it.
4. After finish downloading the file, extract it somewhere, for example in you Desktop or Download folder.
5. Back to NetBeans window, in Plugins window. Navigate to Downloaded tab. Click on "Add plugins ...". Navigate to where you extract the files and select org-icefaces-netbeans-modules-lib.nbm, org-icefaces-netbeans-modules-visualweb-frameworks.nbm, and org-icefaces-netbeans-modules-web-frameworks.nbm under nbms folder on your extracted folder. Then, click on "Install" button to start the installation.
6. During the installation process, a warning about the signing but not trusted plugin will appear. Just click on "Continue". This warning is appeared because this is not yet verified by NetBeans.
7. After the installation of the plugin is finished. Close the Plugins window. Start a new project and verify that the plugin has been installed by looking whether you can make a new Project, Web -> Web application then on the last wizard page (4. Framework), you can see "ICEfaces" and "Visual Web ICEfaces".
Now, that the ICEfaces is installed on the NetBeans, see you on the next day to "play" with it.