I use Excel 2000, but the process should basically be the same.
If you want to separate a horizontal line of objects within a cell, you should select a group of cells under the line of objects you want to separate with this horizontal line.
Once you have the group of cells selected, simply use the toolbar to format the group of cells and add a border to the bottom of those cells.
You can use the print preview to see how this looks.
I have not seen an instance in Excell where you can add a horizontal rule...instead you have to use the process I've described above.
Hopefully, this is what you're looking to do, please let us know, okay?