hmm... sounds like the person that packaged p7zip messed up then? try contacting him...

to make sure you can get a file list of the installed package by
dpkg -L p7zip

and if you are really adventurous, dissect the deb file by:

mkdir junk
cd junk
ar x p7zip*.deb
gunzip data.tar.gz
tar xf data.tar
and if you are lucky you will find the 7z binary in the innards now spilled into the "junk" directory. That is a gross way of going about it but the fastest way to advance: to sidestep the entire packager...

edit: to get "ar", apt-get install binutils