Here is how I got FlexBuilder 3.0.1 plugin installed under MyEclipse 7.0 M1 using Pulse. It should work the same way for MyEclipse 6.5 and Europa.
First, I had to download and install a generic Eclipse 3.4 (Ganymede) from the eclipse.org. Then I installed FlexBuilder 3.0.1 plugin into that eclipse folder. FlexBuilder will not install into a MyEclipse folder (at least the one created when installing via Pulse) because there is no features and plugins folder in the top level folder.
The FlexBuilder install creates C:\Program Files\Adobe\Flex Builder 3 Plug-in\com.adobe.flexbuilder.update.site. This update site file is not there until the FlexBuilder plugin is installed and it will not install without an eclipse root folder that it recognizes. This is why you you have to install the generic eclipse.
In Pulse you can install plugins by adding a new update site. So you convert the filepath of the update site created by FlexBuilder to a URL and add it to your MyEclipse profile via Pulse and then install it and you have a working FlexBuilder plugin.
–Chuck