To fit the Wavetrac, you'll need to check the part number of the diff being sold first, then (if it's the right one) purchase the fitting kit from Wavetrac.
You'll then need to find a garage (or similar establishment) who are able to either cut the diff housing and install the new piece (this requires some fairly serious machine work) or do the same but on a second axle, maybe from a crashed car or similar.
Frankly, if and when I do the diff, it will be via Birds. I'm not arguing (at all) that the Wavetrac is inferior as this process applies to the Quaiffe as well, but I don't believe Wavetrac has ever had the support it needs in the UK. I haven't been able to find a reliable fitter who can quote for and supply the whole thing as well as fit it and THAT is what you need or else it is going to be all on you.
ALL of the garages referred to on the Wavetrac site (in the UK) are VAG specialists and the Wavetrac is a well known application on simple, FWD setups. I have called all 4 of the suppliers listed last time I checked and NONE had any experience or interest in working with the Wavetrac products for BMW's.