There are two different answers depending upon whether you are relying upon Java applets (the 'Run' tab) or the new BC2NetLogo.
The BC2NetLogo program should restore the previous model automatically.
To restore the previous model you worked on when not using BC2Netlogo (using the 'Run' tab) you'll need to look in your browser's history for the URL when you last used the Behaviour Composer. (The Behaviour Composer used to keep cookies to automatically resume where you were last time but many users complained about this. Perhaps we should restore it as an option.) In the future to avoid needing to look at the browser's history you can bookmark the URL.
To create a new model (and keep your old ones) use the 'Create a new empty model' button in the My Models tab.
Howard was answering the question 'if I'm using BC2NetLogo and I want to import a model that I created without using BC2NetLogo'.
I hope this helps.