hi sleepyhead. I was there on a sunday and was told that there were no tours that day.
Anyhow, I heard that they have hourly tours. And then there are the paid tours that includes mushrooms cooking.
It's best to call before going down, and be very, very specific about what you are asking about. I called that sunday before I went and was told that they are open, but weren't told that there were no viewing of the farm that day. Only the main office was open, with some mushrooms up for sale.
I found another site, you can check this out too
http://www.mycofarm.com.sg/