7zip is usefull sometimes, but has nothing to do with scribus. it's only to open the .tar archive – you've been advised to download it because in your operating system there was no standard application selected for this archives.
there is no reason (or i don't know any…) to install 1.3.9 – use 1.4.0.rc6 instead! it's the new stable release. (at least almost – there is still one bug that blocks the final 1.4-release.)
utnik