how can i Select a face loop in zbrush (ZModeler )?
Select to what end? Zmodeler is made up of a series of actions and targets so generally all you need to do is define your target as polyloop to apply your selected action to a face loop.
change your green Mask selection tool to lasso. (which is CTRL + Shift)
press CTRL + Shift on an edge and it should select that edgeloop.