To get rid of infamous M clunk in BMW Motorsport differential, you need to open it in order to install RacingDiffs rebuild pack.
This is the special tool required for opening the LSD unit which can be found following BMW differentials:
E46 M3 (2005 - 2006, CSL),
This unit is opened by unscrewing the 6-point collar opposite from crown gear. Pay attention that it is counter thread, so you are unscrewing it clockwise.