How can I fill the seams between our concrete walkway and driveway?

The solution is simple... seal the seam with a flexible asphalt crack filler such as Blackjack Speed-Seal Blacktop Filler. It's not hard to do and will last for many years with proper preparation.

The first step is to, if necessary, open up the crack so it is at least 1/4" wide and at least an inch or so deep. The crack filler is less likely to fail if it is firmly seated in the crack, as opposed to simply putting it over the surface as you would apply tub caulk! You can also use a grinder if you're careful. If the crack is already wide enough but just messy, you can use pretty much any tool that will fit into the crack to scrape out loose material... old screwdriver, prybar, chisel, "goth" fingernails, etc.

It isn't necessary to wash the surface and a little remaining dust isn't a problem. Also vacuum around the crack so the filler stays clean while it dries.

Blackjack Speed-Fill because it will fill cracks up to an inch wide. Some brands cannot bridge a gap that wide because they shrink too much and will pull away from the edges, breaking the seal! Cut a 1/4" opening in the end of the tube to give you good volume. You'll want to pump the caulk as deeply as possible into the widened seam so work slowly and deliberately to allow the thick filler to push into the crack. It is not necessary to fill the seam level to the surface. In fact, you will find it neater to have the caulk a slightly lower than the surface during this initial application.

Once the crack if filled, use your finger or a tool to smooth and press the caulk into the seam.

You could also try polymeric sand. Clean out the seams as best as you can (garden nozzle on jet setting), fill with the sand and water it down. It will harden and form a strong flexible bond and can be used anywhere the sand will fit into. Here's a link:

http://www.pavestone.com/polysand/

Thank you for your suggestions. this sounds good to me. thanks for the link. Ruth