Unfortunately Bryce cannot make objects physically appear and disappear. Like the previous writers mentioned you need tricks. The transparency one works very well but if you don't want it to fade in and out you need one more trick. Make sure that the last frame before the object is supposed to appear is keyframed so there is no transition effect between 100% trans and 100% vis. I had to use this many, many times and although it it a royal bugger it works.