I'm playing with the software Tajima Pulse, and I think I'm missing a step in how to use the density line feature. I've drawn a box and used a complex fill. 

I changed the density to linear increasing, clicked on the density line tool.  I get the arrow will a little bead on the tip, but it won't let me do anything.  I get the universal no sign the red circle with a slash through the middle.

What am I missing?

Thanks,

This option only in version 14 and level Maestro.