Q. Why is magick spelled with a "k"?
A. The 'k' is used by some Pagans (but not all) to refer to true magick, to differentiate from stage magic.
Modern usage of this spelling is attributed to Aleister Crowley, who supposedly added the 'k' for numerological reasons as well. 'K' is the 11th letter in the alphabet, and 11 is a powerful master number. From a numerological perspective, 11 represents intuition, creativity, and inspiration. Qualities that he felt were suited to the word 'magic'.
Also according to Crowley, magick is defined as "the Science and Art of causing Change to occur in conformity with Will." I think that sums it up nicely.