Some days ago my Eclipse was working fine and a +/- appeared in every block that could be collapsed (functions, classes, etc.)... but now it does appear, and I don't know how to activate that feature again. I know this question is kind of silly, but it's driving me nutts.
If it helps, it's Eclipse Helios, running on a Gentoo Linux box.
Preferences -> C++ -> Editor -> Folding ?
Make a right click in the editor window and go to preferences there, then only the editor-relevant section of the preferences dialog will appear. This works for JDT, CDT etc...
Try this option: Preferences > Java > Editor > Folding > Enable folding
You can do Ctrl + Numpad_Divide to enable folding. Also if you right-click on the area where the +/- was supposed to be, you can see there is a folding option.
I assume you are using Java, but look under the settings for your particular language.
Under the Window menu, select Preferences.
Under Java->Editor->Folding. Select "Enable Folding".