they're not scripts.  shrinkwrap is a modifier, and retopo is a tool.  in edit or object mode, look for the modifiers tab and add a modifier.  retopo is a mode. in general, for any feature and as for any software, i suggest consulting the online help files at http://wiki.blender.org/index.php/Manual.  information about the shrinkwrap modifier is at http://wiki.blender.org/index.php/User:Jaguarandi/SummerOfCode2008. information about the retopo tool can be found at http://wiki.blender.org/index.php/Retopo.

scripts, however, are easy to use.  just pull up the script menu, select one, and select it.  you might have to read to find out if you have to have something selected when you begin, but everything i've messed with so far just required clicking on buttons.