"Select" components such as <h:selectOneMenu> or <h:selectManyListbox> allow you to select one or more items in a component.  The JSF components <f:selectItem> and <f:selectItems> allow you to specify which items can be chosen.

How you select the items in a JSFUnit test depends on whether you use <f:selectItem> or <f:selectItems>.  For <f:selectItem> the values are explicitly set in your JSF page and it's easy to click on a particular item in your test:

<h:selectManyListbox id="Weekdays">
   <f:selectItem id="selectMonday" itemValue="Monday"/>
   <f:selectItem id="selectTuesday" itemValue="Tuesday"/>
   <f:selectItem id="selectWednesday" itemValue="Wednesday"/>
   <f:selectItem id="selectThursday" itemValue="Thursday"/>
   <f:selectItem id="selectFriday" itemValue="Friday"/>
   <f:selectItem id="selectSaturday" itemValue="Saturday"/>
</h:selectManyListbox>

To click one of these items in my test I would do the following:

JSFClientSession client = jsfSession.getJSFClientSession();
client.click("selectMonday");

If you are using <f:selectItems> you don't easily know the id that needs to be "clicked".  So you would Using the HtmlUnit API with JSFUnit :

<h:selectManyListbox id="WeekdaysUsingSelectItems">
   <f:selectItems id="WeekdayItems" value="#{weekdaylist}"/>
</h:selectManyListbox>

 JSFClientSession client = jsfSession.getJSFClientSession();   
 HtmlSelect select = (HtmlSelect)client.getElement("WeekdaysUsingSelectItems");
 select.getOptionByValue("Monday").setSelected(true);