You can just show up and buy a one day race lisence ($7-10) and pay the late fees that they will add to the registration for the race. Your best bet is to preregister, It is cheaper and will save you nothing but time, especailly at the mission race because it is the busyest race of the year and you will spend a long time in line(you can preregister @ the events website which you can find on the cycling bc calander)
Of course if you are going to do a few races buy a cycling bc race lisence. they are about 90 bucks but you can't do Canada cups or Nationals (which are supposed to be in whislter). I found this to be the easiest way even if it cost me more over the whole season who don't need to waste 30min filling out forms every time you race.
Oh one last thing, the off road calander, don't count on what it says now to be what is going to happen. I usually changes a few times between now and the summer