They were used, but because they were trained to think that food was under the tanks to get them to run underneath them and set off the tilt fuse, there were a few problems:
The Russians had used their own tanks for the familiarisation.
Therefore, when fido was turfed out to do the deed, they didn't recognise the strange looking and sounding German ones and prefered the nice familiar ones.
You can just picture it, buffoonery of the highest order. I believe Emporer Mong suggested it first!