Surface Density Charge As Function of Space

Hi,

I have the following problem on Comsol 4.2.

In my model I want to vary the Surface Charge Density depending on the radial direction r.
When I enter it as a function exp(-r) in the "Surface Charge Density 1" the input appears orange.

Does someone know why it stays this way or is it even possible to define an anisotropic surface charge density?

I've attached a screenshot and my project for better understanding.

Kind Regards

Mussie Beian