EditFind and fix issues
Mostly, you will find and fix issues by trial and error. You should play your game to find issues and then examine your actors' behaviors and properties to try to understand why any bugs are cropping up.
Luckily, the Popfly
TM Game Creator editor does provide some help. For performance issues, the performance button will show you the current Silverlight framerate as well as the time between game world updates under "Current Tick Interval:". It also shows you the number of event listeners (behaviors) and actors that are currently active. If your game is slow, it's a good idea to try to reduce both the number of actors and the number of behaviors. Complex motion behaviors, particularly those set to last for a given distance can be particularly taxing, so one tip is to try setting your motions for duration, or for constant motion, instead of selecting "Forever", select "While Receiving Event" and set the appropriate Drag property to 0.
Opening the "Console" window can also help. The Console can help you trace events happinging in your game as well as show you any errors. Errors are especially easy to create when using custom code, so use this window to help debug issues with your custom behaviors.
Finally, the "Edges" button is super useful! This button will draw the edges the game engine uses to decide whether your actors are colliding with each other. Turning this on will help you figure out multi-actor interactions and collision resolutions.
When all else fails, try restarting your browser :).