>>> from arcpy.sa import *
>>> m2w = Con(IsNull("wt1"),FocalStatistics("wt1", NbrRectangle(3,3),"MEAN"),"wt1")
The FocalMean command you're familiar with is not available in the new raster calculator. The documentation you pointed to is wrong and I will submit a defect to have it changed. To accomplish this task you need to use FocalStatistics with the mean statistic type within your Con statement.
It should look like this:
Eric, I was currently solving this problem why it doesnt work for like 2 hours. Thanks alot for helping me!