Ok, seems that I found the answer. I have a namespace in my structure that wasn't defined, and that's why I wasn't gathering data.
As Tomalak said, it's a valid name as it seems. So, the command ends like this:
{$inputRoot_Res/urn:PutResponse/ArrayOfInterfaceObjOutput/xml-fragment/idEntradaCaller/text()}
Where "urn" is defined in the namespaces box.
Sorry for the trouble, and thank you Tomalak and Jens Erat for your answers. =)