Juice has two modes...

In design mode you can create beams and joints and drag them around with the mouse. None of the parts are mechanically connected, so you can move them around freely. Nothing moves unless you drag it with the mouse or change it's parameters with the property window.

In operation mode, the pieces are connected together and everything moves as a jointed structure. You can still click on pieces and drag them, but now you're not really dragging but pulling on them. They don't necessarily stay right under the mouse, it's like you've attached a rubber band to them instead. (Said rubber band is a little too bouncy for my liking, but that's on my to-do list.)

You start off in design mode. To switch to operation mode, press the "play" button on the toolbar, or use the Mode | Play menu item. To switch back to design mode, press the "rewind" button, or use the Mode | Design menu item.

The pause button only affects design mode. When pressed, the physics still work as usual, but the model's main engine is stopped, so the model freezes. It might topple while frozen, but if it does you can pick it back up using the mouse, or you can restart the simulation by pressing rewind and then play.

I find it helpful to design the model just above the floor, so it drops slightly. If it drops too far, it may bounce out of control; if it's actually penetrating the floor, it will be launched skyward rather forcibly. A short drop (perhaps one linear unit) usually allows it to settle gently.

When entering operation mode, the model's main engine will be paused for a couple seconds, with the model posed in mid-stride. This helps get any initial bounce out of its system before it starts to walk. You can adjust the pose delay and the pose position in the model settings dialog box, which you will find under the Options | Model menu item.