If you go to C:\Program Files\Scirra\Construct\Plugins
Is there a ShadowCaster.csx in there? C:\Program Files\Scirra\Construct\Plugins\Runtime should have one too.
If they aren't there a re-install of construct will most likely fix it.
Well, other than I didnt install it in the default location, yes there is a ShadowCaster.csx in /Plugins/Runtime. It has a file size of 27kb and a date of 07/07/2010. I did try reinstalling once and that didnt fix it.