Ideally you would remove the entire pad (two per rotor) and apply to entire surface that will come into contact with rotor. The friction that occurs on this surface is typically where the squeal comes from in most instances.

In my case, since I ended up not doing brake job and replacing the pads, I applied only to the area that I could easily get to without removing them. That would be both the North and South points the brake pad as you look at it on the caliper, grabbing the rotor. My intention was to do a brake job that ended up not being necessary and fixing the existing squeal was an afterthought. ( I had a new set of Pagid pads that I ordered from BavAuto.com and had applied the anti-squeal 24 hours ahead of time, which is suggested, but not required. This would have prevented the future squeal in my case had I swapped out the pads and done a brake job).

Ideally, you would jack the X5 up and leave on jack stands, remove all 8 pads, apply anti-squeal to all 8 'outer' surfaces ( the surface that will actually be grabbed by the caliper), let stand for about an hour or so, and re-insert to each caliper. If this is done, you will most likely eliminate your brake squeal. You can call the techs at BavAuto on their 800 number. They are pretty helpful and all have done this to theirs BMW's, and can walk you through and other questions/ concerns. PS- it's nice day today in Boston so if you have the time today would be a good day outside. The actually labor involve would be 15 - 20 minutes per wheel after it is jacked up, to remove the pads; and about 5- 10 minutes per wheel to re-insert.
Hope that helps. Also, if your looking for 4 front Pagid pads with them already applied I'd be happy to cut you a deal- live in Boston.