<field name="mycategory" type="category" extension="com_content" label="Select a category" description="" show_root="1" />

<extension version="2.5" type="plugin" group="content">
    <name>a</name>
    <author>a</author>
    <creationDate>a</creationDate>
    <copyright>Copyright (C) 2010 Open Source Matters. All rights reserved.</copyright>
    <license>http://www.gnu.org/licenses/gpl-2.0.html GNU/GPL</license>
    <authorEmail>admin@joomla.org</authorEmail>
    <authorUrl>www.joomla.org</authorUrl>
    <version>1.0</version>
    <description>A Plugin</description>
    <files>
        <filename plugin="testplugin">testplugin.php</filename>
    </files>

    <config>
        <fields name="params">
            <field name="mycategory" type="category" extension="com_content" label="Select a category" description="" show_root="1" />
                  </fields>
    </config>
</extension>