Many users believe that they have to have separate software to open zip files that are attached to emails. This is not the case. Lotus Notes - the versatile software that it is - will open and extract the zip files from the Notes client.
To open an attached Zip file, simply click on it and select the view option. The file(s) in the zipped attachment will appear at the top of the screen. You can then click on the document and Notes will display it. To extract the file, simply right click on the document name (or click on Viewer in the menu bar) and click on Extract files. It will then let you extract the files to a directory on your PC or network.
This is a basic feature in Notes that seems to be overlooked by many users.