There are several ways to control the playout of an element, does not matter what kind of element it is.
Load the element first
The most common way is to load the item into the timeline and to press the play button in the workbench or the timeline after that.
Load and play
In case there is currently no item loaded on the particular layer, the load and play can be executed with a single keystroke using the alt-key. To load and play an element:
Make sure the layer the selected element will use is not used at the moment
Click on the element so it is selected
Press the alt-key to load and play the element
In addition, you can also doubleclick a movie in the workbench which will immediately execute the action defined in just:live's preferences. In such case the options are:
Load and Cue
Load and Play
Only movies can be cued, so in case Load and Play is used as the default double click action, the Cue will be ignored and the non movie element will be loaded only.
Usually each layer has to be started explicitly. However it is possible to execute actions on multiple layers at the same time by pressing the command key while clicking the first element of each layer. The play command will then be issued to all selected layers.