Well, like a lot of things in Motion there are several ways to do stuff like this.
If you're using a Retiming Behavior like Set Speed, it actually has Ease In and Ease Out parameters. And if you are using the Timing controls in the Inspector itself, you could go two ways:
1) You can adjust the Variable Speed keyframes in the Keyframe Editor by editing them and/or adding additional keyframes and accessing more functionality via: either Opt.-Click or RT mouse click to bring up contextual menu, then Interpolation>Bezier,Ease In, Ease out, Exponential, Logarithmic, etc. So with the interpolation set to Bezier, you can adjust the handles to get desired ramping effects.
2) You can of course add Parameter Behaviors like Ramp to the Retime Value parameter in the Timing controls of the Inspector as well.
(You can actually add both keyframes and behaviors and I think Motion averages out the effects of both. Either that or it self-destructs your Mac like 1960s sci-fi tv computers with lots of sparks and explosions; either way it will be entertaining to watch.)