I've run into this problem quite a bit, and it came up on a post about Ray Camden's ORM Scanner.

So the conditions are:

• User tries to install extension
• Install option does not work
• They unzip the file
• User imports the extension
• The extension works

Two subtle pieces of information that don't always get reported:

• Extension user on Windows
• Extension author on Mac

This particular problem doesn't appear if:

• Author uses PC
• User uses Mac

The cause of your woes:

.DS_Store files.

I'm not entirely sure why.

To combat this I build all of my extensions with ANT. (I know we provide a packager, but the work flow doesn't work for me.) Here're the relevant ANT targets:

<target name="build.copy.files" description="Create a installable version of the source code.">
<echo message="Copying Files to Build Location"/>
<copy todir="${build.dir}" preservelastmodified="true">
<fileset dir="${dev.dir}">
<exclude name='settings.xml'/>
<exclude name='settings.properties'/>
<exclude name='.project'/>
<exclude name='.settings'/>
<exclude name='build.xml'/>
<exclude name='**/.DS_Store'/>
</fileset>
</copy>
</target>

<target name="build.zip" description="Creates a zip file of the build.">
<echo message="Creating Zip File"/>
<zip destfile="${package.dir}/${app.name}.zip" basedir="${build.dir}"/>
</target>