It's the Route 77 sticker you need. This sticker is found on the big rotating wheel in the scrapyard section.

Also, play the level again when you have it, and right at the start, when you're driving from the desert into the Metropolis, you pass a roadsign. Slap the Route 77 sticker on this as well to get two sticker collectables.

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I think it's unlikely that the number of stickers you have will impede you being able to install Creator Pack 1. However, just in case, the things you can delete to reduce size of profile are:
- Stored levels on your moon - Backup to PS3 HDD or USB stick so you still have a copy somewhere;
- Photos from photobooths (I set the collection option for these items to off)
- Photos you've imported with Eyecam - if you have any.
- All custom items you've picked up that you don't think you'll need
- Saved costumes

That should keep your profile to a usable size if it's causing issues.

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