To be able to select it as a behavior, there are two methods:

1. (permanent - always updated) Go into WED's preferences and for updating of externally modified files, choose "Auto" instead. This way, any edit you make will be updated automatically in WED. You may need to close WED and reopen it and your level though.
2. (temporary - works once, but has to be redone in a future case) Just close WED and reopen it and your level.

"You level up the fastest and easiest if you do things at your own level and no higher or lower" - useful tip My 2D game - release on Jun 13th; My tutorials