commandLink is not rendering in a pageBlockTable cell

Question

I have the following code:

<apex:pageBlockTable value="{!nuevaBitacora}" var="bitacoraParaInsertar" id="table">                
            <apex:facet name="Agregar">
                <apex:commandLink action="{!agregaBitacora}" value="Agregar" id="idAgregar" rerender="idBitacora"/>
            </apex:facet>
            <apex:column headerValue="Fecha y hora de inicio">
                <apex:inputField value="{!bitacoraParaInsertar.FechaHoraInicio__c}" />
            </apex:column>
            <apex:column headerValue="Fecha y hora final">
                <apex:inputField value="{!bitacoraParaInsertar.FechaHoraFin__c}" />
            </apex:column>
            <apex:column headerValue="Tipo de actividad">
                <apex:inputField value="{!bitacoraParaInsertar.TipoActividad__c}" />
            </apex:column>
            <apex:column headerValue="Descripción">
                <apex:inputField value="{!bitacoraParaInsertar.Descripcion__c}" />
            </apex:column>
            <apex:column headerValue="Evento">
                <apex:inputField value="{!bitacoraParaInsertar.Evento__c}" />
            </apex:column>
        </apex:pageBlockTable>

I can see the fields, but I can't see the hyperlink to add the new record:

Answer 1

Similar to Keith's answer, the important part is the value of the name attribute on the facet tag.

If the 'add' link is supposed to be on each row rather than rendered in the header cell:

<apex:pageBlockTable value="{!nuevaBitacora}" var="bitacoraParaInsertar" id="table">                
    <apex:column>
        <apex:facet name="header">
            <apex:outputText value="Agregar" />
        </apex:facet>
        <apex:commandLink action="{!agregaBitacora}" value="Agregar" id="idAgregar" rerender="idBitacora"/>
    </apex:column>
    <apex:column headerValue="Fecha y hora de inicio">
        <apex:inputField value="{!bitacoraParaInsertar.FechaHoraInicio__c}" />
    </apex:column>
    ...

Answer 2

The facet names are fixed by Salesforce to things like "header" and "footer" (depending on the surrounding apex tag which looks for specific names) so the first thing to try is changing this:

<apex:facet name="Agregar">

to this:

<apex:facet name="header">

See the bottom of the apex:pageBlockTable documentation.