Ah, ah, ah...only so many free answers ;) You'll have to actually crack open the manual and look this up (in the Help menu). Try typing in 'next keyframe' and you'll be pleasantly surprised. And the keyboard shortcuts for menu commands (such as what you're asking) are right there in the menu with the command itself.
Also- if you take some time with the tutorials you'll see Motion is actually designed around behaviorial animation which is a lot more intuitive and easy than keyframing. Keyframing is there for exacting detail and as a kind of last resort but if you take the time to learn, you may find you rarely need them.
The reason I need keyframing in Motion is of course for those times I need "exacting control" over specific attributes and functions that would require less time than "playing around" with the app. After awhile all the keyboard shortcuts for FCP, PPro, AE, Combustion, Boris Red, Shake, Soundtrack Pro, DVD Studio Pro, Lightwave, 3DStudio Max, Maya, Particle Illusion, Mirage, Vegas, Acid Pro, Sound forge etc. all kind of become a mind-numbing blur and to have to look up yet another shortcut in yet another 300 page manual while trying to quickly finish a client project (started by the client in Motion) becomes quite a chore. That is why I was desiring a quick answer from those who have used Motion more than I have. The reason I like the Cow is I can get quick answers to specific questions in all these programs, and I also readily help others who don't know other apps as well as I do. Thanks for the help, guys..