For windows, we don't package SFCGAL in the PostGIS 2.1 release. You need to use PostGIS 2.2 which should be released this month.
You can use the experimental builds now on winnie and help us test PostGIS 2.2 (would be very appreciated)
The CREATE EXTENSION approach for SFCGAL came in PostGIS 2.2. So that is the other reason your CREATE EXTENSION doesn't work.
Unfortunately you can't mix SFCGAL packaged with PostGIS 2.2 with PostGIS 2.1 in the same database since the functionality is embedded in the postgis-2.2.dll and having 2 postgis dlls running in the same database would be not very pleasant. So though in theory you can, trust me that very bad things will happen if you try it :).
You can however runs PostGIS 2.1 in one database and PostGIS 2.2 in another. so they can coexist fine on the same postgresql instance. That's actually how I test.
On a related note, I will be updating the zip with SFCGAL 1.2.0 as soon as it is released (which birds told me should be soon) -- which will fix several issues involving straight skeleton and some other functions --
Hope that helps,