Summary
To walk forward or backwards, just press the controller in the direction your character is facing (Towards) or in the opposite direction your character is facing (Back) respectively. To crouch, hold any of the three Down positions on the controller. And finally, for Jumping, you can jump in any of three directions: {{ 
}} for a straight up jump, {{ 
}} to jump backwards, or {{ 
}} to jump forward.
Details
Marvel Vs. Capcom 3 is all about movement. Your characters are rarely going to be standing still and the key to success in this game will come down to how well you can maneuver your character around the screen. And while, at higher levels, you will be spending most of your time Dashing and Super Jumping and Flying and Air Dashing and what-not, you'll never get anywhere without first growing accustomed to the three most basic forms of movement: Walking, Jumping, and Crouching.
It should be noted, though, that M.O.D.O.K. actually does not possess a Jump! Whenever you hit Up on the controller when using M.O.D.O.K., he goes into low flight mode instead.
Strategy Corner
For those of you players who are coming from a Street Fighter background trying to get into Marvel Vs. Capcom 3, it should probably be worth noting that walking will actually play a hugely diminished role in this game than in Street Fighter games. In fact, you will probably be spending the majority of your time Dashing, Jumping, Super Jumping, Air Dashing, and just about every thing else that's not Walking. So if find yourself spending most of the game just walking around, stop. Learn all the other forms of movement instead.