I've got an Ico sphere. First I marked the seam:

Then I Selected the whole thing and UV unwrapped:

However, when I go into the UV image editor, nothing comes up:

I'm using cycles
I've got an Ico sphere. First I marked the seam:

Then I Selected the whole thing and UV unwrapped:

However, when I go into the UV image editor, nothing comes up:

I'm using cycles
You're on the Render Result image in your UV editor. Render results don't have texture coordinates. Use the Image menu in the UV editor to load an existing image or create a new image.