What I'm saying is that you HAVE to use the whole string. What it is doing is telling the system to take the current angle and add 90 degrees. So if the current angle is 0, it will add 90, making it 90. If the current angle is 90, it will add 90, making it 180. You can also use the minus sign to subtract 90.
In your case it would look something like this (where what is in quotes is what you actually type, not the quotes)...
On touched TouchRight, system --> set layer 0 to "LayerAngle(0)+90" degrees
On touched TouchLeft, system --> set layer 0 to "LayerAngle(0)-90" degrees