Sorry, right now you can only play courses or categories from the app's Download Lessons section. In the meantime, you can use remote-access apps like Splashtop Remote to access your PC and use your iPad to play the customized lessons that you created in Little Reader.
In case you haven't seen the post about Splashtop yet, it's this one:
http://forum.brillkids.com/little-reader/little-reader-available-on-the-ipad-today!/