One of the quirks of the Eclipse IDE is that it doesn't like working with files that aren't in a project. There's no way to just open any arbitrary text file. This makes it a bit difficult for developers like me who are used to being able to open up any of the headers in the Palm OS SDK for reference while developing.
One way to get access to the SDK headers is to add a symbolic link to the SDK header folder into your project. To do this, right-click on the project in the C/C++ Projects window, and choose the "New/Folder" command. Give the folder a name, like "sdk_includes", then click the "Advanced >>" button. The window will get larger, and you should check the "Link to folder in the file system" checkbox, and then navigate to the appropriate SDK include folder. For a default installation, you'd use "C:\Program Files\PalmSource\Palm OS Developer Suite\sdk-5r4\include" for an Palm OS Garnet project or "C:\Program Files\PalmSource\Palm OS Developer Suite\sdk-6\headers" for a Palm OS Cobalt project.
If you create this linked folder, do be very careful. Unlike a CodeWarrior project, where entries are just references to files, deleting the project entry will also delete the file on the disk. The tree you see in the project window is a direct reflection of the contents of your folders, and the only safe operation is to delete the original link, rather than individual items.