The M3 liners won't fit your fenders, so you don't need to use the mounting bracket on the duct. If you search the P/Ns on ecs tuning you will see pictures of the actual ducts.
I used 3M automotive double-sided tape to fuse the ducts to the bumper, and it was rock solid. There is a pretty general DIY on here that I found when I did my research, so keep searching.... It's buried in here somewhere.
I really wish I could have gotten it to work, but the e90 must be tighter in that area than the e92 - that's the only explaination I can think of to explain why I failed