Since a couple of versions, the possibility to bind a shortcut to indent and de-indent selected text is missing. Unless I'm looking in the wrong place, I can't find it anywhere in the Shortcut Mapper.
By default indent and unindent should be mapped to Tab and Shift+Tab, respectively. If you want to modify them go to Settings->Shortcut Mapper... and select Scintilla Commands at the top. In the list should be SCI_TAB and SCI_BACKTAB.
Dang, it's that easy, isn't it? It might be a good idea though to add (De)Indent to the list of commands, since some people might want to bind those functions to different keys.