You'll need to edit the file:

    <OPENVPMS_HOME>/update/archetypes/org/openvpms/archetype/workflow/act.customerAppointment.adl

and add the status.

Change it to the following:

         <node name="status" path="/status" type="java.lang.String" minCardinality="1"  defaultValue="'PENDING'" >
            <assertion name="lookup.local">
                <propertyList name="entries">
                    <property name="PENDING" value="Pending" />
                    <property name="CANCELLED" value="Cancelled" />
                    <property name="CHECKED_IN" value="Checked In" />
                    <property name="IN_PROGRESS" value="In Progress" />
                    <property name="BILLED" value="Billed" />
                    <property name="REBOOKED" value="Rebooked" />
                    <property name="COMPLETED" value="Completed" />
                    <property name="ADMITTED" value="Admitted" />
                </propertyList>
                <errorMessage>Invalid Appointment Status</errorMessage>
            </assertion>
        </node> 

Save the file and reload the archetypes using the archload script located in the <OPENVPMS_HOME>/bin directory.

You will then need to restart OpenVPMS for the change to appear.

Note: you will need to manually apply this change each time you upgrade OpenVPMS.

-Tim