The cache is a small magnetic keyholder. It may be a bit hard to find as there are quite a few hiding spots. Hopfully the page title will help a bit.
Logs are on Rite in the Rain paper. If they appear damp or wet, wipe with a napkin, paper towel, your pants/shirt, etc. and/or let it air out for a moment or two. It should then be good for signing.
Be careful when you're searching as not to hit any of those "sharp spots". The cache is hidden in an area that doesn't have any of the sharp ones. You may be able to see it if you look close enough, but watch out for mugglers, as I'm sure there will be a lot of them out here soon!