What I've found to be working is to set up the property in Swift as either an optional or a an implicitly unwrapped optional:
The first one basically means "this property might be a label or it will be nil" and the second one means something like "this will be a label at some point". By setting it up like this, you're not forced to initialize the properties in init(). You can read more here, under Optionals: Swift, The Basics.
I'm not sure which one is to be preferred here (I'm really new to this to), but I guess it depends on the situation.
You'll have to set up a corresponding Doc root var in Spritebuilder as usual, of course.
Hope this helps.