I tried using the Spawner GUI at first it worked but then something happened (this was back in 1.301) and it stopped working. I've now got the 1.4 and I tried using it and it still doesn't work, whenever I right click the spawner my Minecraft crashes.
I use mod manager, and not many mods work for me, but one that does work is TooManyItems. (I duno if this is relevent or not)
If there is a reason to this i'd be great if you can get back to me.