Here is an xml you can use as template.
I use an external xml for the combobox for example named combobox.xml and ad it as include to the main xml.

In the main xml you need also to ad the action to each scene like this.
<scene name="scene_2" onstart="box2();" >


<krpano version="1.0.8.15">

<plugin name="combobox" devices="desktop|tablet+android" zorder="10" alpha="0.8" native="false" cbfontstyle="bold" cbfontsize="12" itempadding="10" itemfontsize="16" scale="1" url="plugins/combobox.swf" alturl="plugins/combobox.js" align="topcenter" x="0" y="7" width="330" rowcount="15" height="25" keep="true" onloaded="fillbox();"/>

<plugin name="combobox" devices="ipad" zorder="10" alpha="1" native="false" cbfontstyle="normal" cbfontsize="11" itempadding="10" itemfontsize="14" scale="1" url="plugins/combobox.swf" alturl="plugins/combobox.js" align="topcenter" x="0" y="10" width="350" rowcount="15" height="20" keep="true" onloaded="fillbox();"/>

<plugin name="combobox" devices="iphone" zorder="10" native="false" cbdesignscale="1.5" cbfontsize="16" itempadding="5" itemfontsize="11" scale="1" url="plugins/combobox.swf" alturl="plugins/combobox.js" align="topcenter" x="0" y="5" width="370" rowcount="15" height="30" keep="true" onloaded="fillbox();"/>

<action name="fillbox">
additem(' Apresentação Video Depoimento', loadscene('scene_1',null,MERGE,BLEND(2)); );
additem('Presidente Sergio Machado', loadscene('scene_2',null,MERGE,BLEND(2)); );
additem('Presidente LULA', loadscene('scene_3',null,MERGE,BLEND(2)); );
</action>

<action name="box1">
plugin[combobox].selectitem ( Apresentação Video Depoimento));
</action>

<action name="box2">
plugin[combobox].selectitem (Presidente Sergio Machado));
</action>

<action name="box3">
plugin[combobox].selectitem (Presidente LULA));
</action>

</krpano>


Hans