xs:extension

The elements you add in XSD extensions go after the elements in the extended complex type. So if your XSD looks like:

<xs:complexType>
    <xs:sequence>
        <xs:element name="BaseElement" type="xs:string/>
    </xs:sequence>
</xs:complexType>

<xs:element name="TheExtension">
    <xs:complexType>
        <xs:complexContent>
            <xs:extension base="MonitorConfig">
                <xs:sequence>
                    <xs:element name="ExtensionElement" type="xs:string" />
                </xs:sequence>
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>
</xs:element>

then valid XML would look like this:

<TheExtension>
    <BaseElement>somevalue</BaseElement>
    <ExtensionElement>someothervalue</ExtensionElement>
</TheExtension>

The extension definition looks unnecessarily complicated, but both complexType and complexContent are necessary (at least for the JAXB compiler).