Hi Nancy,
Generally clematis should be fertilized when the new leaf stems are an inch or two long and the ground temps are in the mid-50s. I usually sprinkle a handful or two of bonemeal around the base of each plant and add mulch, being careful to keep it away from the crown and stems of the plant. If you're still getting snow, I think I'd hold off for a while - you don't want to promote new growth if you're still getting freezing temperatures.
