Try vaseline and socks or bag balm you can get at any drug store.

Can you describe more in detail as to the type of crack. Some dogs pads do this normally. As long as they don't actually tear into the tender flesh of the foot or if she is not showing any discomfort.

If they are tearing and starting to seep or bleed, you need to get her into the vet. Pads are tough skin and require suturing to heal properly. They do not heal properly on their own, and are highly susceptagble to infection.