I just downloaded that one, lol.
I think I will just forget about signing messages, since it gives me a headache, I am trying for two hours now and am getting sick of it.
because you can't import an address using it. multibit classic allows importing address.
also, looks like you can import the address to blockchain.info wallet even though you already have that address on another account. you can use this method to sign a message.
it's just that the balance won't stop loading if you imported a duplicate address (you can still delete the imported address after you sign a message. but don't delete the other one.).