I think I've got my nick registered with gribble at the OTC. Does this mean that it is registered in the WOT or is that a separate step?
Well, the IRC nickname is separate from the WoT username (i.e., they don't have to be identical). Once you've registered a username with gribble, you are in the WoT.
How do I get a rating or rate someone on WOT?
Anyone can rate you once you are in the WoT. To rate someone else, you need to have at least one rating (that's an anti-spam protection so new accounts aren't sending out bogus ratings to each other).
I tried to add the OTC ID to LocalBitcoins.com, but it says I need to " Sign it using the bitcoin address you registered with the #bitcoin-otc Web of Trust (WoT)."
You probably didn't add a bitcoin address to your WoT profile. Use ChangeAddress to add a bitcoin address. Then use your client (e.g., Bitcoin-Qt) to sign the message that LocalBitcoins is asking for.
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http://wiki.bitcoin-otc.com/wiki/Beginners_Guide#.3B.3BchangeaddressI thought that bitcoin addresses were supposed to be single use. I generally try to generate a new address for every transaction.
You never need to send or receive bitcoins from this change address. If someone sees your -otc WoT profile and sends a donation to you at that address, you would receive the funds but wouldn't know from whom.
LocalBitcoins is only doing this as a way of proving that the -otc profile you are linking is truly yours. I suppose they could have used GPG signature but for whatever reason they are using Bitcoin message signing instead.
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http://localbitcoins.blogspot.com/2012/08/now-you-can-connect-your-bitcoin-otc.html