lol. i just checked with my classics meg. curved part does need to be on the bottom in order to fit into the waist cavity.
dont stress dude.
1) tap the pin out on the torso part of that curved piece (heres a tutorial on how to remove pins http://www.sector70.com/forum/tm.aspx?m=12735).
2) flip the torso around and put the pin back in
3) tap the pin out where the entire trigger section connects to his 'butt'. this will allow you to access the 3rd screw on his waist (you may also want to unscrew his legs first just so you can have better access to this pin if you are finding it difficult to tap the pin out).
4) remove all 3 screws on his waist to disassemble it, rotate the curved piece to the correct orientation
5) put everything back
done.
FYI, this can definitely be done, as i've fully disassembled my classics megs for a custom paint job.