D3D doesnt allow decals (mine didnt, anyhow), try running OpenGL or SoftWare.
Also, to 'flip' a decal, you have to manipulate the face it is on.
Say you have some blood on a wall, and you want it rotated/the other way - then make the face it's on -1 on the X axis - this will mirror it.
The change wont be seen in-hammer, but ingame it works.