I think you need to correct your indent for onClick method.
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()