no mate just wack it on, the arm will just compress to where it wants to be as you tighten it up. just get a bolt in as soon as you can then that will help hold it as you get other one in.
It's nigh on impossible to get the arm under the cam, if you do manage it, you won't be able to do up the bolts, so if it's being a pain to align with the holes, then pull it back out and give it a wiggle.