to add to others' comments, i believe that you can only redeem airmiles.ca points for rooms, not convert them to spg points.
i have done this in the past. you must call spg and reserve with the airmiles code corresponding to the certificate that you will receive when you redeem your points.
not all hotels have availability for that rate code.
you can see the number of airmiles required by going one step further into the redemption process on airmiles.ca. from memory, they're something in the vicinity of 1300 airmiles for a cat 4 and 1500 for a cat 5.