Reading pixels from texture2D (how to make texture readable?)

Hello, I need to **read a texture's pixels at runtime**, and I'm getting the "**texture is not readable**" error. I know I should simply mark the texture as readable in the Texture Importer Inspector, but I can't find any checkbox that allows me to do that, and am getting very confused. What am I doing wrong? What should I do? I'm using Unity Pro 3.5.0f5. Thanks