Couple of questions..
First, have you created a ScriptInfo.xml file for your script as described in the
Scripting Documentation?
If you have created that and it's still not working, are you zipping the folder itself, or just the script files?
If the files are within a folder inside the zip, Plus! will not recognise it as a valid script, so make sure you only zip the files.
I can't think of any other reasons it could fail atm, but if it still doesn't work, post it here as an attachment and I'm sure someone will take a look and try find out what's wrong
Hope this helps.