From what I've read, you can't really do anything about your percula. They will either take to the anenome or they won't. Hopefully not the later.
I've had a similar experience and it took to the anenome within a week and before I got the anenome, I had the percula for about 5 months.
I usually feed it a piece of shrimp once a week.
As for it being under a rock, there isn't much you can do. If you move the rock, the anenome will just move to where it is comfortable. Just be patience with it and the percula and if it doesn't work out for you, you atleast have a beautiful invert in your tank