Question as stated in the title: how to install the GlassFish Server Adapter on Eclipse Helios 3.6?
At the time of writing this (07/26/2010), there are in theory two ways to install the GlassFish Server adapter: via the Eclipse Marketplace or via an update site.
Update: As mentioned by Thorbjørn in a comment, the adapter is now (09/24/2010) available in the Additional server adapters dialogue (the "normal way").
There are several ways to do this but one ways is to right-click in the Server view, select New > Server then click the Download additional server adapters link and select the GlassFish adapter.
Go to Help > Eclipse Marketplace... and search for "glassfish". Then click on the Install button of the GlassFish Java EE Application Server Plugin for Eclipse and follow the steps.
Go to Help → Install New Software, Add the following temporary1 update site: http://download.java.net/glassfish/eclipse/helios. Be sure to uncheck Group items by category and you should see the Oracle GlassFish Server Tools as shown below:
Click on Next and follow the steps.
1 The GlassFish plugin is being moved to the Oracle Enterprise Pack for Eclipse (OEPE) and will be available at http://download.oracle.com/otn_software/oepe/helios/wtp in the future (see Bug 312912).
The problem mentioned above is actually not specific to the GlassFish plugin and is not platform specific. The issue is a p2 bug in Eclipse 3.6 when using a shared install (which is the root cause of the problem) as I discovered after reading this other question on SO and the Bug 322929. The problem is related to a p2 bug which is fixed in Eclipse 3.6 SR1 and after installing Eclipse 3.6 SR1 RC4, things are now indeed working as expected. Yeah!