Acho que você precisa corrigir seu recuo para o método onClick.
class ZoomToSelectedFeatures(object):
"""Implementation for Test001_addin.btn1 (Button)"""
def __init__(self):
self.enabled = True
self.checked = False
# Implementation of OnClick method of Button's class
def onClick(self):
# Get the current map document and the first data frame.
mxd = arcpy.mapping.MapDocument('current')
df = arcpy.mapping.ListDataFrames(mxd)[0]
# Call the zoomToSelectedFeatures() method of the data frame class
df.zoomToSelectedFeatures()