Its a bit of a pissed up design mate, i had to do the following on mine:
For a start you need some RTV silicone to form the gasket seal between the flange and the manifold, also i found that the bolt holes didnt line up properly and were at a propper pissed up angle!
I ended up sloting the holes on my manifold flange to get the bolts parrallel, also use a fair amount of silicone on the joint.. as you tighten the de-cat to the manifold it pulls itself square by that olive jobbie!
If you want anymore help mate give me a PM, all in all i wasnt too impressed with the design esp. from a magnex
