Read this post for how to add the button code below to one of your toolbars. The button works fine for me, when I select several files, even with spaces in the names. This is the button that defaults the archive name to that of the Source folder.
<?xml version="1.0"?>
<button backcol="none" display="both" textcol="none">
<label>Zip</label>
<tip>Copy selected files to .zip archive in source folder.</tip>
<icon1>#addtozip</icon1>
<function type="normal">
<instruction>@set ArchiveName={dlgstring|Enter an archive name to zip selected files to:|{sourcepath$|nopath}}</instruction>
<instruction>Copy ZIP HERE CREATEFOLDER="{$ArchiveName}"</instruction>
</function>
</button>