Using 1/8 bit, how do I simply plunge down to make a 1/8" hole? (In Carbide Create)
You can't. It doesn't support drilling --- you have to enlarge the hole by ~10% and cut it as a pocket.
Or pocket it "true size" using a 1/16th cutter.
I asked the same question starting out for quarter inch holes and felt dumb when I realized for most woodworking .25" and .252" are for all purposes the same.
Another option would be to use a tool which does support peck drilling --- MakerCAM is one such: http://www.shapeoko.com/wiki/index.php/MakerCAM
In know you're specifically asking about Carbide Create, but if you're willing to try out Fusion 360 it's very good for plunging down to drill holes.