I have version 6.0-4ubuntu1 installed, but here ftp://ftp.info-zip.org/pub/infozip/src/ i see only version 6.0.
If you want the current version (that you have installed) first check what package we're really talking about:
Not all binaries belong to a package of the same name. This one does.
Of course that requires you to have the source repositories enabled but that's simple enough to fix if you don't.
To compile it you'll need
And then you can follow the standard instructions for the application you're compiling (usually some form of