I know installing the guest additions in 4.1.8 is extremely easy (did it yesterday) by following these steps in the Mepis Wiki:http://www.mepis.org/docs/en/index.php?title=Virtualbox#Guest_Additions
Not sure for 4.0.10. I was able to choose from VirtualBox 4.1.8 or 4.0.16 by adding the same repo Gaer Boy is speaking of to my source.list file.
EDIT - For the record, that repo does have 64bit packages in it. Also, if you are wanting to install the guest additions in windows, there are exe files in the same location as the linux files.