The Slice tool will divide an image into sections. These sections are not temporal, that is they cannot be time or frame based. They are physical structures, so you can make the bottom half have a URL displayed, and attach the HTML code to make that URL a clickable link. But this is all handled by HTML code. When you use the Slice Tool in combination with File > Save for Web and Devices, your output is not just the GIF file, but a full HTML web page that places the images (each slice will be a separate image file) so that it appears as a single image, with separate areas having URL link code attached.
What you are trying to do, have a link attached to a single frame of animation within a GIF, is not possible. The GIF file format doesn't support anything like this, it does not support clickable links, nor does it support special behaviors per frame. Clickable links are entirely handled by HTML when it comes to image files. You would have to use Flash for something like this, but you would be creating an SWF, not a GIF. Fireworks and Illustrator would work the same as Photoshop in this regard.