Open notepad:
1.Go to your "receive coins" button,click it,copy & paste that addy in notepad.This is YOUR wallet addy,any BTC sent to it will end up in your wallet & no one else's.
I only have one receive addy,if you have more than one...........I don't think it matters,just pick one.
Someone correct me if I'm wrong2.Now,go to "transactions" button,click it, find the transaction you want to sign,right click on it & click on "show details".Copy & paste the "Transaction ID" in notepad.
3.Your ready to sign a message,go to "file",upper left corner,click it & click "sign message".
4.Copy & paste YOUR wallet addy in line 1.........In the body copy & paste the "Transaction ID".Put your info in there too,what the BTC was sent for & how much (EXACT amount),name,shipping address,etc...
5.Click "sign message" on the bottom,it will generate a unique ID #.Now copy & paste the info from all 3 sections in an email or PM,keep them seperate in the email (don't just bunch em together) & volia!!!!
You just signed a message

point 1.. This worked for me the first time, then I looked an ass

, always double check the blockchain after sending a transaction to confirm your address before signing an message, It wasted so much time for group buys chasing payments..
if you think you have got it, you can do this in reverse. VERIFY A MESSAGE
Address : 1DononEbNw8traGfgQpvkV99tRPkat8H7t
start message-----------
This is the message southerngent signed.
end message------------
Signature : HCjjSUOpvoK7HkSvwqc687hHiLwfw8GEaV2wmNuAg8ErQ6Mb58JAhFtuyC9OonjUm9nBs5kbilaaxWc
8Nff4c7Y=
Try it.. then when you have signed a message you can verify it. Only you can sign a message with your address but anyone can verify it..