There ks no such thing as "install a zip file". Could yolu please try to explain exactly what you are trying to do? Copy a zip file to your phone? Create a zip file? Extract a zip file? Install a zip or unzip program? (For the latter, install "InfoZIP's Zip" or "InfoZIP's UnZip" from the repositories. Keep in mind that they will give you only the command-line utilities, no GUI.)
Do not remember the name (and have my N900 not with me to check), but there os also some app listed in HAM (unzip) that integrates in file manager and unzips zip files automatically on click...
What I know is I have it since the early beginning. And it worked/is working without any problems ...
(for sure I now use more cli, but it is still nice for fast unzip/look/delete (all from any file manager).