In the tutorial I'm using it says that after I make a model I can add behaviour scripts to it but when I try to there are none there, and I did that new script thing in Map Properties, what do I do to make behaviour scripts appear in the list?

You need to include the templates files in order to have behaviours and actions to apply. When you add a new script in map properties you must check the type of templates you will need: A5_template, A6_template, Shooter_project, but not an Empty_script.

I check those, but it still comes up with nothing when I try to give a model an action.

Also check Preferences->Advanced->Reload of externally modified->Auto.

Still Nothing.

I have the same problem, I am apparently able to make a floor be mirrored (in the tutorial) but it doesnt show any options. They are all greyed out.

- In WED->File->Map properties->click the folder icon to add
your script(.WDL)

- Save the map file other than untitled.wmp

just save the map (important)
then you can add a new script with standard actions

I had the same problem too and when I saved the map and added the model
it worked

It still won't work, I've tried all the ideas suggested, then I right click a model I've placed, hit properties, behaviour and when I click action there are none there...

Try adding the action yourself. Just type the name of the action (case sensitive) in the field yourself and run the level. It should work.