I just tried this and it's fairly simple and pounding on the keyframe button gets automatic within a few strokes. Pretty easy, really.
BUT... big but...
The effect is not really convincing. It works, yes, but it's obviously not the text entry you would see on an ancient Wang word processor. The movement and blinking of the cursor box layer are completely linear because the type-on effect is linear. If there is a way to sync the movement and blinking of the cursor box with a variable type-on effect in Motion, I admittedly do not have the chops to figure it out.
I performed the same operation in After Effects with my Image Lounge filter and there is simply no comparison. Image Lounge also lets you add raster lines and other screen effects right inside the filter. And you can set the effect to mimic realworld typing with pauses and speed changes.
Image Lounge is an expensive toy even if you have After Effects but it is completely appropriate to this particular task.
bogiesan