The Thule ProRide bike carrier would have come with fittings that work with T-track aluminium bars. You need to use the three T-headed bolts, these bolts slide into the channel that runs the length of the roof bar, you usually have to remove the rubber strip first. Spare T-track fittings are available if you no longer have the parts that came with the bike carrier - click here for details.
Alternative fittings are available if you have BMW branded bars, which have a wider channel - click here for details.