HybridGull,

If you have Atmos elsewhere, I think all you have to do is copy 'ambient.syn' from its current 'synth' folder to the corresponding 'synth' folder for MEP MX (maybe then entire synth folder contents?). Locations depend on operating system -- in Win XP stuff is usually in C:\Documents and Settings\...\Magix\... while in Win 7 I think its C:\Program Data\... I've not got my editing machine here to check. 

Regards,

Carl