There are some sample ASLX files here:
http://www.quest5.net/downloads/samples.zip (though they haven't been updated for Beta 1, so may be broken...)
Also there are two demos in the source code. Go to
http://quest.codeplex.com/SourceControl ... /view/8050 and then browse to trunk/WebPlayer/Examples
There will be some better examples available later on in the beta process - eventually there will be a full tutorial, just as there is for Quest 4. But I need to finish writing the software first

Regarding your object questions:
- You can make an exit locked by clicking "Locked". To unlock it, give the exit a name, and then create a script to set the "locked" property for that exit to false. (Eventually there will be a nice easy command you can choose to unlock an exit, and you'll be able to select the exit from a drop-down)
- Using keys - use a script as above. (Again, this is something that will be made easier eventually by improvements to the Core library)
- To print a message if an object is too heavy to take, on the Inventory tab leave the "Take" option as "Default behaviour" and ensure that "Object can be taken" is off, then just enter the message in the "Take message" box.