First, I want to thank Bananeweizen for providing the core piece of the answer above. Basically, since MyEclipse 10.5 and earlier does not ship with the Eclipse Marketplace Client pre-installed the first step is to install the client from the: Help > MyEclipse Configuration Center... > Software tab and clicking the '+' icon to add it to the MyEclipse configuration... (if you head over to our blog we have the images there to show you: http://www.genuitec.com/blog/?p=4461)
Next, follow the wizard directions and allow MyEclipse to restart after the installation. Once restarted, you can add items from the Help > Eclipse Marketplace... item as you'd expect.
Now from the Eclipse Marketplace Client you can add whatever software you'd like, like the Cloud Foundry plug-in you asked about.
After the installation and restart, you can add your Cloud server by opening the WTP Java Extras perspective by using Help > Open Perspective... and selecting the "Show all" checkbox...
Then to add your cloud server you can right-click in the WTP Servers view and select 'New > Server'
And select VMWare > Cloud Foundry and follow the steps in the wizard. And that's it.
In closing, since you had to ask the question, it's obvious that we can improve this process a bit, and we will. So that it will be a bit more obvious how to pick up plugins from the Eclipse Marketplace, we'll be adding the Eclipse Marketplace Client to the default configuration of MyEclipse 10.6. I think it will be a nice enhancement for all our users. Thanks for your support of MyEclipse!