It's possible it's inside the wall. Sometimes you can cam into the wall and try to see it. Also if the house is on your land if you click on the Land option and go to objects you can see a list of all the avatars with items on your land, click your name and items belonging to you will then have an outlined glow. Go to the wall where you tried to put the decal and see if you see it's outline there, if you do then yes that where it went!
I took them up on their discounted premium membership just so I could get in on this, now you're telling me they changed it at the last minute and it's open to anyone? So I basically got shammed in other words. Come on guys! Don't you ever plan or think things through first?? Figure out what you want to do then stick with the gameplan!