Manually add items to dropdown

[Matt]

I want to manually add items to a dropdown, and keep getting the message,
"The specified value is not valid".

The field in the XSD file (main data source) is as follows:
<xsd:element name="is_this_part_of_a_cluster_or_web_farm" default="1">
<xsd:complexType>
<xsd:simpleContent>
<xsd:extension base="StringType">
<!-- <xsd:attribute name="_isNull" type="xsd:boolean" /> -->
</xsd:extension>
</xsd:simpleContent>
</xsd:complexType>
</xsd:element>


"StringType" is declared in an include file as:
<xsd:simpleType name="StringType">
<xsd:restriction base="xsd:normalizedString">
<xsd:minLength value="1" />
</xsd:restriction>
</xsd:simpleType>


Is this poorly formed XSD? Can anyone tell me what the _isNull attribute is
doing?

Thanks