Download the mod from your packages area.
Check out the package-info.xml
It should be something like this
<?xml version="1.0"?>
<!DOCTYPE package-info SYSTEM "http://www.simplemachines.org/xml/package-info">
<package-info xmlns="http://www.simplemachines.org/xml/package-info" xmlns:smf="http://www.simplemachines.org/">
<id>vbgamer45:SMFTraderSystem</id>
<name>SMF Trader System</name>
<type>modification</type>
<version>1.0.1</version>
<install for="1.1.1,1.1,1.1 RC3,1.1 RC2">
<readme>ReadMe.txt</readme>
<modification type="file">smftrader.xml</modification>
<require-file name="Trader.php" destination="$sourcedir" />
<require-file name="trader.template.php" destination="$themedir" />
<code>smftraderinstall.php</code>
</install>
<uninstall for="1.1.1,1.1,1.1 RC3,1.1 RC2">
<modification type="file" reverse="true">smftrader.xml</modification>
</uninstall>
<install for="1.0.7,1.0.8,1.0.9,1.0.10">
<readme>ReadMe.txt</readme>
<modification type="file">smftrader107.xml</modification>
<require-file name="Trader.php" destination="$sourcedir" />
<require-file name="trader.template.php" destination="$themedir" />
<code>smftraderinstall.php</code>
</install>
<uninstall for="1.0.7,1.0.8,1.0.9,1.0.10">
<modification type="file" reverse="true">smftrader107.xml</modification>
</uninstall>
</package-info>
Update the file and rezip the package and place it in your packages folder and it should unstiall then.