I’m not sure what you mean by “click on fill.”
If you’re trying to create text with the regular “text” tool in Inkscape, and you want this text to be filled in with lines, the most straightforward way to fill the text is to use the Hatch Fill extension. You can read more about it here:
http://wiki.evilmadscientist.com/Hatch_fill
If you’re using Hershey Text, there isn’t anything to fill, since the letter shapes are composed of open lines.
I hope that this helps. If not, can you please explain very clearly what it is that you’re trying to do, and what it is that is not working?