Ron, I just so happen to be cooking this exact recipe right now. My salmon went on the WSM 20 minutes ago! But enough about me...
If apple is all you have, it should be fine in place of alder. Use the same amount as in the recipe, (3 large chunks and several smaller ones) and the results should be virtually the same. Apple and alder are both medium to light smokieness/strength. If all you had was something like hickory or mesquite, you may want to use much less.