Pretty hard to find it in the last years. First of all you should find the MSC session report when the new amendments were adopted on the IMODOCS website (Meeting documents > MSC ). Choose the correct session and read through looking for IMDG code. In 2016, the 96th session the MSC 96/3/Add.1 will show the IMDG amendments. Not quite! Will refer to a circular letter, No.3598. Now google for this letter, hopefully you will have the link from somebody.
Actually, ... give it a try from were you can access, because the old links doesn't works anymore.