Is this a variable that takes in an already spawned actor? Try placing this Blueprint in the scene and access the Variable over the Property Window at the right side of UE4. There you should be able to assign something to this variable.
EDIT: Here are some pictures of what i mean. Made a test Struct with an Actor Reference:
http://puu.sh/dKw1L/e75967108c.png
I can’t set the Actor in the Blueprint default at the bottom. So i dragged it into the scene (spawned it) and on the right sight i can fill the struct:
http://puu.sh/dKw4U/98a3685b1e.jpg
Does this help you?