In Slic3r 0.8.2 you find it in "Print Settings" - "Generate support material". Last time I tried (several versions ago) it was still quite experimental, and to quote the changelog
(much work is still needed on support material), so don't expect too much.
Personally I design my parts to be printable without support, or add support struts manually in difficult cases. Still, for some parts generated support structure is probably the most sensible option, and I second NelsonRap with skeinforge being the solution in those cases.
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-Nudel
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