The first kenning is describing the claws of Grendel. Basically, it is an extended simile written backward. Think this way: "The heathen's hand-spurs (claws) were most like the steel of hardened nails." It is helping to build the enormity and strength of Beowulf's challenge, and the actual kenning is the "heathen's hand spurs" part.
The second, "heather stepper" describes a deer, which is more obvious thinking of a "horned stag." Heather is a plant a deer walks through and if hungry enough will feed on, so a heather stepper describes a deer.